You can start by calling local animal control, and borrow some traps. Don't feed them - bait the traps and put them in the crawl space (if they fit) or in the garage. The food goes into the traps. If your girlfriend wants to do rescue, she needs to do a bit more research here, and set up a place for them that they cannot get out of. Animal Control will not come out to do this, you need to do it yourself. They only come out to investigate animal cruelty cases, or dogs running at large. If the one cat is stuck somewhere and cannot get out, you will soon have a dead cat.