Actually, I think we hold that purely al pi halacha, there is no reason that he CAN’T divorce her (last mishna in Gittin). It’s strongly discouraged, of course, but there’s a 3-way machlokes and I think we go the third way.
I believe the only time he has a chiyuv to divorce her (assuming, of course, that it’s a legal marriage) would be if she has been unfaithful to him. If he’s a Kohen there could be other situations as well.
I hope that this is a purely hypothetical question; if it is being asked for any specific reason I hope that everything works out as well as possible for all involved.
Hatzlocha.