I'm with 33girl. If they were
together just
30 days after you and him broke up - It makes me feel as though they were 'courting' before you were even officially single. That idea alone just doesn't sit too well with me.
And then the fact that she ever so graciously forgave him for hooking up with you.
It all sounds a bit too suspect for me. I'd chalk it up to just being cordial, in order to save face. Honestly, if you she was really your best friend - she'd never have been with your man...especially not that soon. Not to say that she wouldn't have those feelings, BUT that level of respect would restrain her from acting out.
ETA: And shame on him for poking between friends - so not cool.