Rosalind
Wow, seriously? I mean, I guess their blood would be wicked magical. And normally Healers are pretty good but if you were really desperate and couldn't go to Mungo's for some reason...
Potter doesn't really seem like he's on the brink of death though. At least, not anymore. But if he was roaming around out there for ten years, who knows how unicorn blood works? It could be addictive or wear off unless you keep drinking it or something. So I guess it's plausible. The other possibility would be if one of the escapees got hurt really badly. Rosier's making himself visible but we haven't seen anything from Lestrange for a while, have we? Either way, the list of possible suspects can't be very big.