What do I do if a dog I walked has fleas?

I took a dog for a walk today and within 20 minutes of being home my mom found a flea in our house. We have a dog of our own who I naturally pet when I got home and I would hate for her to get them. We have never seen a flea in our house nor on our dog so I'm confident that the flea did not come from her. How do I politely tell the owner that I think her dog has fleas or do I Just leave it alone?