I think we all cope with these sorts of things differently.

I think you can find a way to be supportive without buying into the delusion that a cure is likely.

And while a cure is not likely, its not 100% out of the question because medical advances are being made every day. You can never say never.

I think if you were to be the voice of reason, and tell him that a cure is not likely, he would see you as unsupportive. Perhaps you could keep your true thoughts inside, that the reality is that she is most likely not going to recover, while giving your boyfriend and his daughter kind words of support----but without lying.

Its a fine line to walk, and I wish you the best. Hugs.