You don't know what the full truth of their relationship is from a social media post, so going straight to "you're ruining his life" was pretty egregious. First off, it's not her fault so blaming her for the fallout is misplaced. Second, they're already involved so he's going to be deeply affected regardless of what happens, and breaking up to leave her to deal with it alone could ruin him anyways. Third, using social media to encourage people to end relationships is simply wrong on it's face, it's not the place to be encouraging strangers to make life altering decisions.

You don't know what the full truth of their relationship is from a social media post, so going straight to "you're ruining his life" was pretty egregious.