I would tell him - sorry.
But tbh if she gets pregnant it's just as much his fault... Any guy who doesn't want to unexpectedly become a dad should be using protection!
Hope she knows what she's in for doing this! He may 1. Walk away leaving her to be a single parent - very difficult! 2. Could get really angry and make life hell for her for god knows how long!! 3. Hang around and do the right thing, but if was that kind of guy she wouldn't be trying to hide this from him he'd be going along with it willingly!