How is it a "green flag" for him to only approach her when she's with her friends?

There's a girl in our dance troupe who recently met this guy, but no one really knew how to feel about him, because they hadn't met him before, and she always felt self-conscious around him. But when he suggested if hanging out with her and her friends would be more comfortable for her, that was literally the one thing that made them changed their minds and consider him a "green flag"

Doesn't sound like much of a flag, desu