Tell you daughter to say something like
"Man I bet Jane and Jill would love to get a mocha late', when do you want us to go?"
If he persist she shold tell him in a joking, but serious way...
"Joe...you know I see you as a cousin"
or
Tell her to retract her offer, and that she dosen't want to date him.
I am having a hard time putting it in kid terms, because I am quite blunt (always have been), so I've never had any confusion in the i'm-your-guy-friend-but-i-want-to-date-you department. My mom was always the nice one....me...I am like my grandmother
!
ETA.
I didn't mean to put kid terms, without asking how old your daughter is. sorry.