Just when you think you’ve seen everything…

New one for me today. Need to know what the proper call is. For context, u11 girls travel soccer. Neither team was great. White team consistently fouled on throw ins by not keeping rear feet down. So on one throw in set to take place in front of her own team’s bench/area, a team mate comes up from behind and places a hoot on top of the thrower’s rear foot to assist with keeping her foot planted. Legal or not and why? Assume if not proper restart is throw in for non offending team yes?