If he is abusive like she says then I'm sure she would have reported.
I am almost certain that an awful lot of abuse never gets reported. I did not report a single one of the instances of abuse, including the two times I was raped in relationships (because, how do you prove that you definitely told them no and they did it anyway; I freeze rather than fight, so little sign of struggle etc.), the time my ex husband strangled me I covered the marks up and didn't report it (for fear of losing everything; we worked in the same job which he would have lost as a consequence, and people talk), the many times I was emotionally abused and sexually coerced, because you just cannot prove it.
One of my exes was arrested twice on accusations of rape, but never convicted, had 2 other girls tell similar stories, still works as a teacher because on paper he's innocent and still has a bunch of people believing that he is just incredibly unlucky with girls accusing him left, right and centre of sexual abuse. He was sexually coercive and had a massive temper on him, but could be oh so charming. I didn't report, because I was with him when he got arrested the second time on a historical case, but let go again and managed to spin a tale. I left him soon after that because I saw what a predator he was, and no doubt I am just another crazy ex now to his new girlfriends.
I have thought so many times about contacting potential new girlfriends of his and of my ex husband's, but with the former I would just run the risk of re-focusing his energy on me and with the latter, he has me over a barrel with the kids, so I can't do either. He has already alienated me from his family and all shared friends, so who would believe me, anyway?
So, OP, please tread carefully. It takes a lot of guts to come forward and speak about abuse for fear of ramnifications. Please do not tell him about the message, but be extra vigilant from now on. I hope that, in your case, it is not true, but please be careful.