My seatmate has digestive problems causing unpleasant smell. What ought the cabin crew do?

Unfortunately this is mostly a case of bad luck. The usability of your mom's seat was significantly reduced but in a manner that's mostly subjective. That leaves it in the discretion of the crew and the crew has the final say.

You are free to complain after the fact and some airlines will offer some form of compensation: typically credit towards future bookings or award miles/points. Rarely cash. If you have the stomach for it you can plow through their carriage of contract here https://www.airtransat.com/en-CA/Legal-notice/Conditions-of-carriage-and-tariffs and see if you can locate the actual policy.

There is nothing your mom could have done to get into business class. Business class is an extremely valuable asset to the airline and most of them will protect the revenue fiercely. They would only give a seat away for free in fairly extreme circumstances and even then only along the pecking order: i.e. upgrade the highest status passenger in the plane and then use the free economy seat to address the issue at hand.

So this is mostly in the discretion of the airline. Air Transat is a low cost carrier with a very mixed set of reviews so the outcome is not unexpected.

After my mom's Air Transat flight took off, her seatmate started to fart stinkingly. She can't tactfully complain to the cabin crew, as her seatmate'd know that she snitched on him.

But it was discreet of her to take her case from the overhead compartment, in full view of the unfortunate, and possibly ill, passenger and go to the front of the plane, without even thinking of asking the crew?

Is it possible that someone who suffers from prolonged and extremely smelly flatulence only disturbs the passenger seating beside them? Smells in restricted closed spaces have a habit of spreading, the passengers seated in front and behind the unlucky person would also be aware of this foul-smelling "wind", especially if the emission of sulphur/sulfur imbued gas was continuous.

If stealing a business class seat, as the OP's mother had done, was permissible, what would stop a second, a third or fourth passenger from doing exactly the same?

In the mother's position, I would have stood up and approached one of the flight assistants and asked if they had or sold bags of peppermints. Purchased one, returned to my seat, opened the bag, popped a mint in my mouth and then invited my fellow passenger to take as many as they pleased.

Peppermint: A common mint given after meals, peppermint has been widely known for helping to settle the stomach and relieve gas quickly. Those suffering from chronic gas related issues may find that the use of peppermint can reduce symptoms during all times of the day. Peppermint contains antispasmodic properties that can help calm the intestinal muscles involved in releasing gas.

One thing your mother could have done, had the plane had passenger-adjustable air vents overhead, would have been to open hers. This would have increased the air circulation around her, more quickly diluting the smelly gases and circulating them away from her.

A web search for the keywords airplane fart turns up some interesting articles, including an incident in which an airplane made an emergency landing after a physical altercation.

The answer given by @phoog is certainly useful and I would try that. There is not much the crew can do if no same class seats are available.

A more effective solution is to apply some menthol cream to the nostrils. This something often done when visiting slaughter houses or tanneries where there is an unbearable stench.

I strongly disagree with the sentiment of most answers. Being sensitive to smells myself, I can fully understand how your mother could not tolerate this situation, and even preferred to stand over sitting in that place.

However, I do agree that she did handle the situation badly. Trying to solve by herself is not a good approach. If she wanted to be discreet, she could have stood up, found the nearest steward or stewardess and approached them, away from the passenger in question.

They should have at least attempted to solve the situation. I don't think bad smells are such a unique event, and I would be surprised to learn they don't have at least some air fresheners to slightly improve the situation.

I also think that there is a limit to tolerance. If a smell is unbearable, you can and should talk to the person causing it. There are ways to handle this respectfully (a question over on IPS might be asked for, and it's a bit delicate, but it is possible).

Legally speaking, when she booked the ticket, she paid for more than just transportation from A to B. The airline is also obliged to provide an expected (i.e. common) standard of service and comfort. This may be very subjective in this particular case, but if the cabin crew does nothing, I would point to this fact and announce that I will not consider the service that was bought to have been delivered unless I can enjoy a minimum of comfort. The cabin crew is typically on a tight schedule especially during relatively short flights, which from your description I assume it was. So they try to solve problems quickly and efficiently, and while they are usually professional about it, sometimes you may have to make them understand that you are not going away until your problem is solved and solving it will be their quickest way to get rid of you.

Well, a stinky fart is unpleasant, it poses no real risks. As there was no risk, there's nothing really to be done. The cabin crew acted correctly, and the best your mother could have done is to mitigate the smell somehow.

Either that or become less sensitive to it. Sorry but go sit in the fart cloud for a few moments and the human brain will filter that stink out for you.

Nothing can be done in this situation as pretty much every action is going to leave someone unhappy and dissatisfied with the situation. Personally I think the way it was handled was rude, selfish and inconsiderate.

From the sounds of the question the flight was pretty full and what should be remembered is that your mom would not be the only person experiencing the problem with the smell from this person. How is it decided that she is able to escape this situation but others are not? Should always remember that maybe there is someone who is going to have more of an issue with situation or more deserving of an upgrade then yourself.

Also need to remember that you have no idea what is causing the flatulence and what the person has done to try and prevent that.