just don't get in their face saying, "smile!" when they don't feel like it.
be there for them.
find out what they like, and do it for them.
just sit in the space with them,listen, and they will smile when they are ready.
I suspect it would be the same way you would eat an elephan: one bite at a time. First step would be to get a professional in the picture to do a proper assessment. Second step would be to let the person know tha tyou are there for them. That smile may take a long time with a lot of work, but if you find the right combination of help and timing, ther emay be potential there... Good luck.
Bring a bunch of things they like to do with you or one of their other friends
Or just let them vent then make a joke about yourself or tell embracing stories about yourself to keep their minds off the heartless
If you caused her to feel that way, then I guess try getting hit by a bus in front of her???? Jokes.