I don't think it would have caused any damage with only 10 hours running like that. Like the posts said, if the fluid was the correct type, just drains some out.
My 2 cents on the loaning/borrowing thing is this. I have a very short list of one close friend and a few family members that can borrow anything I have, except my wife.

The same list of people are who I borrow from. I have had a few minor things happen to things I have borrowed while I had them. I immediatly notified the owner and made the repairs before returning it. Any time I borrow a tractor it is rinsed off and full of diesel when I return it.