There is likely a good chance he'll want it to escalate. As long as she doesn't let that happen, then it's ok - as long as she does tell her boyfriend. By keeping it a secret from him it shows that something extra might be happening that you don't know about. Regardless of whether something extra is happening or not, her not telling him is a red flag as her doing something like this that he might not like behind his back... while it may not be cheating, is also quite bad.