Unless you plan on her being perpetually pregnant, then you're just going to have to deal with them being lonely. Trust me, they'll survive apart. If you know mom is due to deliver in October, then separate them now. Even if the former owners saw a mating, they could still be wrong and it could be sooner or later. If it's sooner, you're risking a breedback. Leave them apart until after the kits are weaned. Putting a male back in with a female isn't always the brightest idea after the female has had kits. She can become extremely territorial and protective. They can attack each other and the kits can end up hurt.
Are these animals of breeding quality? Have they been evaluated by a qualified breeder or judge? If they haven't been, then when the kits deliver, separate them into same sex pairs if the offspring will allow. Female/female, male/male. There is a lot that goes into breeding, but the first and foremost must be that the chinchillas are of a quality to be bred.
As far as prenatal care, good quality food, good quality hay, fresh water, and a kit safe cage. That's all you really need.