The lady was mainly trying to push Anton towards Heath because Arwin "belongs" to the father. She pretty much insinuated that. As you said, we don't know much about Heath, so we really can't say he would be a better partner (or will ever have any kind of feeling for Anton at all and vice versa). The story is just beginning, so it's really hard to say what the characters actually feel. Minus Anton of course, he now seems to be drawn to Arwin with more than just lust.
Also we don't exactly know what's going to happen, but since this is Anton and Arwin's story, it's a higher chance of them ending up together.
Tbh...we don't even know that much about Heath, but I'm pretty sure we know What's going to happen. Because the lady told Anton that Heath would be a better partner to Anton, (but in my opinion) not because Heath is unavailable but because their attitudes probably would for better. Knowing from what I'm seeing already Arwin possibly loves Anton but more like a child, cuz after all, Arwin did do his dad as his very first, and plus he sees Anton as a naive kid who just lusts after him. In my opinion Heath is a better Partner because I feel that Heath could handle Anton better and his naive attitude, by trying to straighten him up and out smarting him, and proving him wrong all the time, which can get him to change, Arwin to me will just make Anton obsessive and lustful