Need advice about my friend's ex!

This is very long but please bear with me. It's a weird situation and I don't want any feelings hurt, but I also don't really want to be involved with her. I am female and in my early 20s. I have been friends with "Bob" for about 5 years now. Bob began dating "Susan" about a year into our friendship. Bob and I are part of the same mutual group of friends (along with my boyfriend): the majority are guys, aside from myself and another close friend. They dated 3.5 years [approx.] I only hung out with her in group settings; we did get along when we hung out as a group, it's just our overall personalities didn't match up (she being extremely high-strung, high maintenance, whereas I am more laid-back and introverted) so we only hung out a few times one-on-one. Aside from Bob, Susan only had one female best-friend.

Last summer, Susan had a mental breakdown, resulting in a short stay at the hospital; she was diagnosed with depression, OCD, and bi-polar disorder, put on meds and attended therapy. It seemed to come out of the blue, we were shocked, but all reached out to make sure she was alright. A few weeks later, Susan came to another hangout and began sharing with me the vivid hallucinations and anxieties she was having and how it was affecting her and Bob’s relationship [Side note: during their entire relationship they only had sex a few times because it scared her too much. It became non-existent after her breakup. Bob shared with my boyfriend that their relationship was now extremely strained as his needs were not being met physically and emotionally].

I told Susan I was there for her if she needed to talk, but over the next couple months, at every single casual hangout with all of our friends, she would pull me aside for hours to talk about it. I felt guilty because I told her she could talk to me whenever, but having her do it at every single hangout made me want to avoid her. I felt even more guilty because she didn’t have many friends outside the group, so I was probably the closest thing she had to a friend at the time. I expressed my frustrations to a mutual girlfriend and she was having the same problem. Susan then also began using her illness as an excuse to emotionally/verbally abuse Bob. She would freak out at him over miniscule things, put him down, scream at him, etc. but would then turn around crying, being apologetic, blaming it on her illness, etc.

We noticed Bob became very quiet at hangouts and seemed unhappy. He broke up with her in the Fall. She had another meltdown which involved some drama with her family, persistent phone calls to him and mutual friends for months, walking inside his house unannounced and uninvited demanding to talk to him, etc. At this point I didn’t want to be involved in the situation, nor did I want to “choose sides”. “Choosing sides” did eventually happen as Bob resumed hanging out with us on a regular basis, and Susan had never gotten particularly close to any of us. After a while, Bob began moving on and “having fun”. Susan found out and began pestering me to set her up with any guy I could find; this made me uncomfortable and I began to not respond to her texts. I found out through a mutual friend that Susan began approaching random men in public asking them to go on dates. This scared me because a) she could be taken advantage of by a strange man and b) she was in no condition to even date. Bob had to change his number because of her calls. I stopped responding to her and making excuses that I was too busy to hang out (as her behaviour was making me uncomfortable).

I still feel guilty I did that, but we were never good friends to begin with and I feel like I’d be hanging with her out of pity. Anyways, today she messaged me again to hang out. I don’t really want to hang out with her at all, nor do I have any interest in beings friends with her. We had known each other for 3ish years but she never wanted to hangout until after their breakup. What should I do? Should I step up & be the better person and hang out with her, or should/can I avoid this without looking like a huge asshole? Advice would be very much appreciated!