It doesn't sound like her problems are actually work related to me. Im no doctor but when one person just doesn't fit in where everyone else seems to be very happy that person is probably experiencing some mental problems. I don't mean to sound mean or like a know it all, but my recommendation is to invite her to express herself to a counselor, that you'll go with her (if you really like that co-worker) so she can get to the root of the problem. If she doesn't want to take such steps tell her politely that she is having a depressing influence on you and if she won't help herself or allow soemone to help her then she needs to limit her conversation about work to positive things not negative.