your friend huh? if this is your friend, why not just ask?
there are far too many apps that uses a heart icon. most of them are for health or fitness apps. so you can most likely rest assures that your "friend" is not cheating on you.
we get this question almost on a daily basis here. the problem is that there are over a million apps with notification icons so unless the app is like facebook or youtube, most of these questions go answered with most of us only giving guesses as to what it is.
so either you can ask your "friend" what it is, or you can be sneaky and look in your "friend's phone" to see what it is.
anyways i wish you luck in your endeavor.