Really there's not much you can/should do. You can try to get along with her or distance yourself for now. There's a good chance the problem will solve itself in time. Personally I think it's a pretty huge age gap but if they really just met randomly and hit it off without pressure then honestly whatever. There are worse things going on in the world than two people under the warm covers of a bed. People are very sensitive about age gaps in modern western society (don't get me wrong, I think that's a good thing) but other concepts do exist and reality isn't always as black and white as we would like it to be.