It sounds like your ds looks forward to it/enjoys it so if I were in your shoes I would continue but cut it down to 2hrs. Five hours is crazy long for a playdate, IMHO. I agree with others to simply tell the mom that for the rest of the summer you need to cut down, no explanation needs to be offered. But have something as back up in case she asks.
It is hard navigating social issues with a dc, I know...I've had to do it myself.
DD1 - 1996
DD2 - 1999
DD3 - 2005
Surfaces are for working, not for storing. - Peter Walsh