I don't know what else he's supposed to do. Law's admitted who he is, and that he's guilty. It's his duty to arrest him.
It's not like when he met Luffy in the bar, when he could at least pretend he didn't know who Luffy was.
All of their everything sucks though. Nami and Brook are weapon dependent, and their weapons don't work, and all of Chopper's striking surfaces don't work.
I'm not even sure Brook can take his hands off his face.
And I doubt any member of the Donflamingo family has no battle ability whatsoever. Even if she's not that strong, she can probably defend herself from people that are incapable of attacking.