Are there any transatlantic cruises between Québec and London, England?

No, there are no such cruises.

The RMS Queen Mary sails from Southampton to New York, however, and she can take the train from London to Southampton, and then from New York to Toronto. Of course, she can repeat the trip in reverse when she's done as the ship sails in both directions.

Note that it takes approximately seven days to make the voyage, so if she does this round trip, this will take fourteen calendar days for travel alone, not counting her train or bus trip between New York City and Toronto. In total, she'll spend sixteen days traveling.

There are slower, more involved cruises between the UK or Ireland and the Canadian east coast, but cruise ships do not get closer to Toronto than Montreal, 552 km away.

Unfortunately, there isn't much demand for transatlantic sea travel, so there few options available.

The Queen Mary 2 is the only passenger ship that regularly crosses the Atlantic, and it usually goes to New York (you can find the occasional trip where it stops in Halifax, but that's not really an improvement for getting to Toronto or Quebec).

There are periodic crossings by cruise ships doing repositioning cruises between seasons, but these are sporadic, one-way, lengthy, expensive, and most do not stop anywhere near Quebec. For example, she could consider the Viking Sea trip from Norway to Montreal this fall, but she'd have to get herself to Norway first, and there's no equivalent trip by sea back. There are occasional repositioning cruises from New York to Quebec, so it's possible she could combine a QM2 trip with one of those, but such cruises run around 10 days, not competitive with the train. In short, there can be limited ways to get there if you don't care about dates or practicality, but the options are pretty limited.

Finally, some freighters do offer very limited passenger space. This is decidedly not a cruise, but it is travel by sea without the creature comforts (you won't find medical facilities on board either). You could try contacting an agency that deals with such bookings, such as Maris to see if any such trips exist. A Great Lakes trip could get her as close as Cleveland, though getting to Toronto from there is still a pain.