The trouble is that unless someone has that exact app it's unlikely you'll get a definite answer. There are literally hundreds if not thousands of apps that use heart shaped notification icons, including dating apps (which I guess is your concern - these questions usually are about that) but also fitness apps, notifications that someone has received a "like" on some app or other, or things that happen to have "heart" in their name (which can cover a very wide range).
So you may be lucky (or unlucky, depending on the answer) but it's more likely that the only way to know is to ask him/her.