Re: What to do ?
I am curious as to the outcome of this. Looking at it from an instructors POV and a business owners POV, no sexual activity should be tolerated in relation to the dojo. This is why companies have a no dating policy, because of questionable situations like this. I think dojo's should have a similar approach, and have a non-dating policy. People go there to train in a professional environment; if things go bad, illegal, or abusive, the dojo as a business/school is associated with this crime. It should not be tolerated and should be dealt with quickly, you do not want something like this looming over a school or a business. It will shy people away in fear of that happening to them.
If this person was put on probation, he must have done something serious. In that case, it was at the risk of the instructor to let him back. Knowing that he is up to something again should be a red flag to terminate his membership permanently. It is bad for the dojo to carry that student, and the student should be spending his time in therapy to cope with his sexual abuse. If all else fails, jail.
I hope you told the instructor. The student should not be allowed to use the dojo for his poaching grounds on victims, and it will be disappointing to your training if you can no longer train there because of the association of Aikido to this person. All in all, it is bad to keep this person in training at the dojo.