What to do with a new client whose dog lunges and barks at other dogs?

I've been in your shoes. I don't promote training services. So, when the client came to pick up their dog, I thanked them for letting me care for them, shared something nice about the time spent, and then told them about the lunging and pulling (I would not mention the marking). I explained that I thought their dog would benefit from someone who could train them during walks, so as to be not a safety danger, and that wasn't something I could provide. I reminded them that Rover Customer Support can try to help them find that person for no additional fee.