Tranquility,

Don't share a lawyer... Most good lawyers would balk at it- it breaches ethical codes to represent both parties. If your lawyer is offering to, find a new one.

Also it would be a good idea, if the cheating bastard (sorry for language) has agreed to give her anything, get it written down, and signed, witnessed by you. That's good evidence if he reneges later... it'd be harder to prove if just verbal...