I don't think that is Google Fit. It's not installed on any of my devices, but whenever I've seen the icon the heart has been upright rather than tilted through 45 degrees and there's a bar across the left-hand lobe (it always looks a bit like a folded paperclip to me, though I doubt that was what they were going for).
The trouble is that there are so many different apps, and so many different types of apps, that use heart notifications that unless you get lucky and someone knows that specific one it's very hard to search it up. It may be simpler to just ask the person in question.