I sprayed them with a natural flea spray called Vets Best and it helped a lot but didn't get rid of all of them. Ugh I don't want to let them in the house..
I'd contact the owner immediately to let them know. I believe all will need a flea bath. You'll need to know what if any treatment they're on, allergies, etc. If for some reason, you can't reach the owners, call Rover support next. If there's any chance that the fleas or any eggs are now in the house, you'll need to wash every rug etc. in hot water in the washer, clean the floors, vacuum the rugs and empty/toss the vacuum filter outside immediately after. And I'd recommend keep checking and brushing all dogs meticulously because even treated dogs can have a flea hitch a ride until the flea bites them.
Contact the owners and see if they will authorize their vet to give you a Capstar for each dog. It's a tablet that will get rid of the fleas within 30 minutes. Of course, they'll all drop off in your house, so watch that. :( You'll also need to ask when they last had flea prevention and what brand. Some stuff can't be given too close together.
So sorry this happened to you! That's awful. I've seen infested dogs come into the clinic, and I just don't know how people live like that!