Hi Mark
I have a few like that as well. Even a couple with upvc frames. If its the upstairs windows I just go over quick and explain to the customers, if it's the downstairs I just wipe over the offending windows with a microfiber.
I did have a really bad one where all the windows leaked. The man was okay but has just moved. When we went he said he'd pass my number on, I said not to bother as his windows are so leaky, he replied that if he closes them tight they don't.
Most people with leaky windows will be used to them leaking in the rain.
Simon