Is it safe to visit Nepal now since the trip is non refundable?

There are three ways your parents can end up not losing the money they spent:

• If they have trip insurance it may cover this sort of thing and enable them to claim a full refund and not go
• If the tour operator can't provide what they paid for, the tour operator may give them a refund. Similarly if the airline can't actually transport them they may provide a refund. This is a reasonable chance but I wouldn't want to count on it. The chances will be increased if the tour agency is part of some sort of association or is licensed in some way.
• If the airline can transport them and the tour agency is willing to take them, then perhaps they can take the trip as planned, or modify it slightly to volunteer in some way and help the Nepalese people recover. I would guess that if the airline and tour people refuse to refund them, it's because it's safe to take them

May 18th is perhaps too far away to predict the state of food and water supplies. Nonetheless contacting the tour people immediately is the way to go. Also look for other sources of information - for example a number of Facebook groups after Cyclone Pam have been great for updates on which Vanuatu hotels and attractions are re-opening when. Doubtless similar groups are being created for Nepal already. Liking the hotels and attractions is great first step, as well as looking for relevant groups.

I'll offer a contrary opinion and say that you're probably fine to go, although you may need to alter your itinerary. Contact your tour operator, they'll be able to advise based on local knowledge, unlike us armchair speculators here on the Internets.

That said:

• Major intercity roads and city thoroughfares will almost certainly be cleared by May, this is a necessity just to get relief supplies flowing.
• Yes, local water supplies may be affected by the earthquake, but since sanitation in Nepal is woeful even at good times, they should be drinking bottled water only and choosing where to eat very carefully anyway. (Also, I infer your parents are Indians, in which case they'll already be very used to this.)
• Since tourists are staying away in droves, the money you will spend as a tourist for hotels, restaurants, taxis, guides etc is more needed than ever.

Update: On June 15, 2015, Nepal reopened 6 of 7 closed World Heritage sites and urged more tourists to come visit.