I'm guessing the worry is about whether people will find them attractive in a boy way if they have big boobs. It might be about seeming too much like a woman if they have such noticeable boobs. There will obviously be people attracted to them, but will those people see their boobs as manly? The literal concern sounds like brainworms, but it seems motivated by dysphoria rather than dating options. That dysphoria doesn't seem based in reason, so it probably can't be reasoned away.

I've heard of treatments that interfere with breast growth, but I have no idea which ones or how effective they are. The acceptance route can have an effect, but if they feel really dysphoric about it, acceptance probably won't get them all the way.