You should look for any reptile shows in your area and shee if she would agree to go with you to one (if no reptile shows, see if there are any breeders in your area willing to let you come see their animals). While you're there you can show her several different morphs and she will be able to handle some and see for herself how docile these snakes can be. She would also get to see some adults there so she can see that a ball python isn't going to grow into a 14 foot monster snake. Just do your research and present her with a facts and make a strong case for yourself. Find out what her specific objections are and address each concern individually.

Is she afraid it's going to escape? Get a secure locking enclosure.

If she is afraid it's going to bite, I will say that anything with a mouth CAN bite. I have been around snakes for many many years. I have owned ball pythons for close to 5 years. I currently own 15 snakes, handle them regularly and have not been bit once. Once you learn to read your snake's behavior, you will know when is and isn't a good time to handle...

Good luck. I had to do the same to convince my wife to let me get them and now she loves them almost as much as I do...