We tried to give my hub's parents $100 when they stayed w/ our kiddos for a long weekend...but they wouldn't take it; so your friend may refuse, just fyi
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How long are they staying? You could either give them a cash gift and/or get them a little gift package like a couple bottles of wine or wine+champagne or something? Or, if she likes to go out to eat, maybe a gift card?
Our pet sitter charges $50 overnight for 1 night, $40 per night for 2 nights, and $35 per night for 3-7 nights...if that helps at all.