I concur with the previous poster, don't take it without permission. Trust me a theft charge can come back to bite you years and years later. That said, how old is your friend? Is she a teenager? Does she live with her parents? If so, you can always threaten to talk to her Mom about it, or you could actually go to her Mom and explain what you know. Just a thought.

If your friend is a working adult then I'm afraid you need to deal with her personally but don't lecture her about responsibility instead try approaching her with a positive, "I'm sorry I went off on you Christie. It's just that I really wanted that snake but we bought it for you so I had to give it up. If you don't want it anymore can I have it?" Something like that.