This is going to seem like a very strange post and believe me I don't know what to think. It's like some storyline from Neighbours or something. I just received an e-mail and have never had anything like it. 
I was friends with a girl last year who to put it bluntly wasn't nice. In fact she was vile to me. She was a mutual friend so I only saw her a handful of times and when I first met her she was always in tears so I was sympathetic. Anyway, her behaviour grew worse and worse. Every time I saw her I'd be told 'who the fuck do you think you are' over the smallest things. If I went round to her house there were always wierd rules like we all had to sit with the lights off. I thought our mutual friends really liked her so I kept up with the friendship until she was just too rude one time and I broke ties. (It was only after a few weeks) It was a bit awkward with the mutual friends but when I asked them about her recently they said they weren't friends with her either because she was starting to threaten to attack strangers when they were with her.
I was quite shaken by her behaviour and relieved not to see her again. I really felt I'd been bullied.
Anyway, I just got the strangest e-mail from her all this time later apologising saying they found a huge brain tumour and that that had been the cause of her behaviour. She said she had thought I was lovely and hoped I didn't think it was an excuse. She's been having treatment the last few weeks so back to her old self.
Now I've never received an e-mail like this IN MY LIFE!! I believe her about the brain tumour and I also know they can alter behaviour. But really, should I believe this?! I saw her being very abusive to her boyfriend and really never saw much pleasant behaviour from her at all. Wouldn't there have been flashes of vileness interposed with the real, nice her if this was the case? I should reply out of politeness but have no idea what to say!! I think if she suggests meeting up I'd shake just at the thought. But maybe she's happy just to apologize and leave it at that.
I appreciate this looks like the oddest post. It's like a soap opera or something. It's not every day that someone says this to you. I've namechanged but I can pm a regular I know if you want to 'verify' me.