Found a Flea during a Dog-Sit, owner wants to hire me again in the future, what should I do?

The key to this going well is all in the way it’s discussed. Yes you should bring it up, in a way that is kind.

For example you could say, I understand you were on your way out when I arrived. I realize that anti-flea medications are not always 100% effective. (This is true-many vets will completely agree) My understanding is that fleas once present in the home will usually continue to lay eggs and the best way to break the cycle is washing of all soft materials (all bedding, blankets, rugs, etc in hot soapy water) and continuous vacuuming of carpets. Is there anything you want me to do or would you be contacting a cleaning service to help mitigate the problem, so that when you return the fleas are continuing to present a problem for you or your pets? (A mention that fleas bite humans as well as animals) Do you want me to take the pets in for a flea dip at a groomer or visit the vet for evaluation?

As for the protection of your own pets, and home, as soon as you enter the door, strip & put all of your clothes and anything else washable in the washer Hot soapy water, then shower and you should be safe. Depending on how it goes it’s up to you whether or not you accept again. If you decline, as long as you do it nicely, (I.e. I’m not the best match / Not your pet’s& home doesn’t meet my expectations) you probably will not risk a bad review.