Get a locking lid(the sliding top kind have a built in lock spot for a little padlock). Then there's no more issue. Without a clearer pic of the injury, it's impossible to tell what caused it, but everyone is correct. A dab of neosporin without pain relief will help and it'll heal up fine, most likely.

I had people feed animals without my permission before(never a snake though, finding a live rodent to feed it is a lot of effort, kind of weird). It's always a bit unnerving, because what if they fed something inappropriate? I mean, your co-worker could have dropped a full grown rat in there, thinking all snakes eat rats, so it's fine right? So a locking lid would be my reccomendation. The tanks are fairly cheap, and nearly completely escape proof. Good investment in my opinion.