This is probably incredibly tactless, but here goes:
You asked WWYD.
I want my child to know that if someone is mean to him he should walk away. That means that I do my level best to never put him in a situation where he can't walk away. And it seems to me that having to visit an aggressive child in a house is that sort of situation.
To an extent your DD will meet aggressive kids over the next few years in kindy and school - but that is a bigger group, and she can choose not to play with them; it's not sustained stress.

So I think you need to ensure that your DD can 'escape' - either by there being other kids, or spaces where she can be on her own, or by not taking her in the first place, or by there being enough adults that someone is always watching your friend's DD like a hawk so she can't cause grief to other kids.

Sorry - we were writing these together - feel free to ignore.

I also wonder if it would help your friend to meet older kids who have been really difficult toddlers - so that there is some light at the end of the tunnel.