I would be concerned about some kind of dermatological condition honestly. The skin looks thickened and red, and its in an odd place for an injury. Has she been bothering at it (licking/chewing) or has she been just walking oddly? It can give you an idea on what you may be dealing with.
Regardless I wouldn't put anything on it without knowing what was happening. Even if some topicals are harmless, it could potentially make the problem worse rather than better without knowing what you're dealing with. I would take her to a vet personally (and I do realize it is a friend's dog rather than yours, but I'd be telling my friend the same thing).