1. A friend that you flirt with, even though you would never date him/her, a practice crush so you're prepared when you actually have one.
"When Simon was my flirt friend and I practiced on him so much that some of my friends thought we were actually dating."
2. A friend that you would gladly make out with if they asked you to, but don't want to ruin your friendship with.
"Eric is my favorite flirt friend."