I've been through this OP, and it is difficult.
My friend's son started being violent and angry and lashing out at anyone and everything around the age of 18 months and it lasted a few years. It was really difficult as we had both just had our 2nd children (on the same day!) and we were juggling and at times struggling.
He did seem to target my son though not exclusively, and at one point my husband suggested we don't see them, but I said feck that and stood by her (she was upset by the behaviour and was doing her best with the skills she had to rectify it, but at that age it really is bloody difficult, there is no reasoning with them)
At one point we managed play group type stuff by just taking it in turns to helipcopter him.
The thing I found most disturbing though was I did start to actively dislke him as a "person" - (and young children are obviously also people, just small ones) and I wondered if I would ever recover from my feelings of dislike for him, which were reluctant because I knew he was so little and it wasn't his "fault" yet I couldn't control or shake the feeling.
I used to say various platitides like "I'm sure he'll turn into a fine young man" to her when she was feeling low about it and you know what - he has. And our friendship has endured.
The phase will probably be short lived in the over all scheme of things - so you need to ask yourself whether she is a true friend you want to have forever or just someone who is around at the moment who you generally like.
It is difficult though.