Heel dents, for the most part, are nothing more than superficial and won't impact your board. There are two instances, IMO, where it's a concern. The first is if the dents are "perfect" for your foot and expanding them would make your stixx less comfortable - tell your friend to ride your cheap board.
I mean really, dropping $600 on a board isn't cheap, I don't think a friend would want to mess you up for the remaining life of the board. I know I'd feel crappy if I did that.
Another instance deals with the stringer. The wood is significantly stiffer than the foam/glass next to it. That is why mostly you dent the foam/glass and the stringer remains at it's original dimension. If the heel dents are approaching the hard edge of the stringer, the glass will rip along the stringer. Typically that doesn't happen on a center stringered board, it happens VERY quickly on perimeter stringered boards, because your heel is close to the stringer on the outside of the board. However, if someone's stance is weird, or teh dents are so pronounced that they are approaching the stringer - it's the cheap board for the friend.