This is a very touchy subject but i strongly feel that you should tell your Sensei. He should know what is happening in his dojo, specially such behavior from a senior student, or anybody for that matter.
Me personally - i'd break his hands unintentionally nikkyo style and I wouldn't have to face the guy for a long while… I hope never to be in a situation, where I will have to make such a decision but if I do, I hope i have the gutts to do just what i said.
sounds like her sensei already knows what's happening.
if law enforcement wont help then perhaps several friends with baseball bats can...
sicko's that prey on women only learn one way. the hard