It just gives the opponent something else to grab—or is that not allowed?
I don’ think it will make any difference. She is only a Blue belt who never competed internationally. Most of her opponents will be battle hardened female Judoka of varying Black belt rank and they won't screw around. Someone is going to stack her on her head within 30 sec and its going to hurt. If someone takes pity on her they will just choke her out.
It’s definitely allowed, so if I was to compete against her, I’d be using that extra fabric to my advantage. It should be easy to choke her out, since if she moves to protect the headscarf, you can use her gi against her (or vice versa).
Of course, next, there will be a rule saying that headscarves aren’t legal to touch during the match or something like that. Although, even if that rule was to come about, I’d still look for a way to dislodge it and use it to obscure her vision.
The smartest thing she could do would be just lay it on her head so that when the match starts, it falls off and her opponent can’t use it against her. Unless they come up with a rule that there’s an automatic timeout when it’s dislodged, in order for her to put her cover back on. Sigh.