My 22mo DS is a biter and has been since he was 12mo, his is caused by teething and frustration/being territorial over toys etc. It's absolutely gutting when he does it, I always apologise a lot, but I can't seem to stop it completely. He's bitten and lashed out at lots of his friends, and fortunately all the mums have been understanding so far.
If the boy is also biting other stuff I'd suggest it's prob teething, the mum needs to do whatever helps whether that's cold carrot sticks or a Calpol dose 20 mins before you arrive, but he may still bite.
If its anger/frustration, you just have to watch them like a hawk and make sure they don't get too close. Helicopter like mad.
Sometimes I can divert him into hugging someone instead, but he wants the 'release' of biting, so I also take hard teething rings out with me and tell him to bite them instead.
I try to give DS lots of positive attention when he's behaving well, then when he bites he gets a firm "NO, no biting, it's hurts" then he's completely ignored either in favour of the other child or someone else until he's calmed down.
So far, all this does is make me and the other mum feel better - he doesn't seem to remember it or feel sorry!
I've started doing the naughty step this weekend, so will use that next time he bites too.
It would probably help if you asked the mum for permission to tell him off when he bites - it really has to be done immediately, and if she's distracted with a newborn she may not be on the ball. All my friends are welcome to tell DS off for bad behaviour, as long as any consequences are dealt with by me.
Re your DD crying, don't worry too much, bites don't tend to hurt for long (DS has bitten me a lot!) so maybe do the quick 'kiss it better' and then distraction route if you're worried she'll get whingy.
Hope this helps