Are you sure the cat is pregnant? My cats have very round bellies when they've eaten. Another common cause of a distended tum is roundworm.
How can you tell whether the females are spayed? If they are more than 6 months old, they would already be mums: are they? Five cats sounds like a reasonable number, and you admit that your neighbour feeds them; are you sure they're not just cats who prefer to be out in the summer?
The reason I'm saying this is that you could be my neighbour: my cats are enjoying the relative summer warmth and I rarely see them except at meal times. They're all neutered - I have 6 - sometimes have large bellies, and like to find a variety of comfy places to sleep, including home.
Is there a chance that you might be jumping to conclusions?