just look in the notification pull down to see what it is.......oh wait.....this is not your phone. you can just ask the person, but i guess that is not an easy thing to do.
just so you know, we get this question every day. and 9 times out of 10 the question never gets fully answered. the problem is that there are thousand of apps that uses a heart icon as its notification icon. and yes there are some dating apps that use it. but majority of the ones i have seen are for health and well being AND NOT DATING APPS. so unless it is something like youtube or facebook, majority of these questions goes unanswered.