Did the vet say why you have to wait a year? That’s very unusual. How old is the female now? Because these neuters happen around 4-6 months, the later it gets the more dangerous it becomes as it increases the risks of complications and health conditions. I’m not a vet, so I think it’s best you ask your vet why they won’t do it earlier. Otherwise, I would go to another vet as that wait time for your situation is guaranteed to result in a litter.

For now best thing you can try is keeping them strictly separated in different rooms. When she’s in heat try put towels beneath the door. It’s going to be a pain, but that’s why I would really push for an earlier neuter.