She's dealing with a whole lot more than just her father's death. Sounds like her entire family is hard to be around. I would definitely encourage her to work on things with a counselor. And sometimes, people just suck all the oxygen out of the air, and we can't breathe. Do what you can, but try not to feel responsible for her happiness.
Maybe it's a good idea to do those things with her that distract her from her problems. Don't respond as much to the depressing things she might say.
I know it seems weird, but sometimes people actually think they need to feel bad. Maybe if she experiences more joy and "fun", she can see that there's benefit in changing how she feels. She may honestly think she's getting comfort when she complains. Hopefully she can realize that not complaining has its benefits too.
Good luck with this Frugalone....it's not an easy position to be in.