@Mary1211 i don't know that I'd ever ask a guest, in a case like this, what they expected to receive as a discount. When they asked for a discount, I'd just say, yes, of course, and then send them $50, with profuse apologies and maybe bring over a bottle of wine or a box of chocolates.
It would be good to have an extra duvet cover around, if you don't already, so it could be quickly changed out if necessary.
The problem with missed cleaning issues, especially the bed, is that when a guest encounters that, it makes them feel the whole place wasn't adequately cleaned, which may not be true at all, but I can understand how it could prompt a general disatisfaction. Maybe your cleaner should have whatever you refund, or at least a portion of it deducted from her pay, to ensure she's more vigilant next time? That's a pretty important thing to miss when cleaning.