I grew up with a family friend's kid who was a few years older than me, who was named Rennie. Her sisters both had super common names along the lines of Anna and Jessica and there was a *reason* her father always wanted a daughter named Rennie but I have no recollection of what that reason was.
Because of her, I kind of like it. I'd love to know the reasoning behind your friend's choice! Renny is not to my tastes but I think there are way way way worse unusual or more "made up" names out there.