That would be so upsetting to me, I would never accept a sit where they breed their dog, that would be an ethical consideration for me and you’re kind of stuck between a rock and a hard place now. If you bring it up to them they can leave you a bad review in retaliation.
I think THS should require owners to specify if they are breeders as they are asking you to look after their livelihood, and that is added responsibility (and potentially liability).
@BillK Good point.