Your first problem is actually getting her here. If they aren't going to get married her own real way of spending any time in the UK is on a visitor visa. Assuming she was granted a visitor visa (which isn't guaranteed) the maximum time she can spend in the UK is 6 months - before the 6 months is up she should leave the UK. If she is in the UK on a visitor visa you could pay for her to have the baby privately, though it won't be cheap.