Consensual non-monogamy can work, but they both have to truly agree that it's what they both want and then they have to spend a good bit of time communicating and being real with each other about all aspects of this. At least he asked her to give permission, and I certainly wouldn't advise her to give it if she really doesn't want to, but I think the reality is, he wants to experience other people and chances are he probably eventually will, with or without her permission. They need to work this through with some facilitation from a therapist or coach who is familiar with this topic.