Yeah, they’re reversible. This is the way, especially if you’re disinclined to stop having sex with her. The sooner the better, also. Oh, and this could be a great way to see if she’s being faithful, if you do it and keep it a secret from her. If she comes to you and says she’s pregnant, and the timing doesn’t match up…well, then.
AFAIK the recovery of a vasectomy can take some time and the guy wouldn't be able to have sex for a while. So while getting snipped and keeping it a secret might seem like a good idea, he would have to somehow explain why no nookie for a period of time. Seems risky. Might as well get one and be honest about it. If she leaves him after that then it wasn't meant to be.