Let me play devils advocate for a moment here.
First off... It was his snake. Not yours. Why should you be mad that he wants it back?
Second... The breeding season is very early on right now. What makes you think that she is already gravid?
Third... Breeding Loans are always a tough one, because of all of the problems that can arise. Why did you do anything without making sure all bases were covered? Friendship aside, business is business. If things can go wrong, they will...
Live and learn.
It just doesn't make much sense to be overly upset about it. Go and buy yourself a female, and keep everything for yourself. That way, you are covered.