(On his part??)

I have a male friend whose been through hell and back with his now ex (and very pregnant) fiance. 2 nights ago, he texted asking me to write of letter about his character (I'm familiar with it because I at one point needed a very similar letter for my child in family court many years ago). He's been open with me about some of their issues and I tell him when he's wrong. And ideas on how to fix it. I thought everything was going a lot better until he asked for the letter. I asked him if domestic violence was involved or any type of assault... As I understand it....if I write this letter that gives the courts free access to call me and/or to take the stand as his witness. So before putting myself on the line...I wanted to know. He asked if knowing would "impede my ability to write the letter"... i said "Yes." He never responded. I am now starting to assume that he never told me the truth about their issues and when she finally kicked him out. Her family is sort of hiding her from him. He originally told me it's because they never approved of their relationship (he is Nigerian, she is Hispanic). But I'm now wondering if I've been advising someone who I thought was a good guy and he may have been putting his hands on her. Considering he didn't answer. Now, I know it isn't my business and I'm not entitled to any details but because he came to me and ask me to write the letter....I can't help but wonder if I've been supporting a friend who is violent.

What would you do? Is there anything to do?