Wait a tic.
Ignore my last post, as it really depends on the situation.
What kind of neighborhood do you live in? If its a quiet neighborhood with not many cars driving around, and not too many bigger animals that want to kill your cat, then it might just be OK to let him outside.
Also, how old is your cat? Honestly, if he looks like hes grown to his full size, maybe a year and a half to two years old, you should let him outside.
If that girlfriend cat of his is a stray, then you really dont want to let it have kittens. Theres a ton of cats around here (I live in a trailer park...) and one of them seems to have had kittens over by a small store down the road. One of my friends adopted one of the kittens, and later found out later that it had FIP. Sadly it was much too late for the kitten. From what I hear, it had died upon his leaving the Humane Society. The reason it got FIP is because the mother cat ran off after having kittens. When that happens, and the cats are getting their milk, they don't build up an immune system. (That is why it is important whenever you have a cat that has kittens, to leave the kittens with the mother for at least 5 weeks)
So, if that girlfriend cat has kittens in or around your yard, I would suggest watching them for a few days and make sure that the mother is taking care of them. If need be, remove them from where they are, and call the Humane Society. Around here, they refuse to go pick up stray animals, I dont know how that works if you have caught them already, but you may have to take them to the Humane Society.
The chances of the above happening, are quite slim. If the girlfriend cat is going to have kittens, it'll probably do it somewhere away from your house. Theres also the chance that the owners/previous owners had the girlfriend cat fixed.