Do I tell???

I guess this goes in this category?

I found out that my brother lied to his wife about how he was able to get a new truck - he told her that he got it himself (he has TERRIBLE credit) and his mom didn’t help get it. Come to find out that his mom did help get the truck. In fact, the truck isn’t even in his name!!! I’m super close with his wife and I feel terrible not telling her, but that’s also not my place? I’m conflicted because I think she would tell me, if it were my life. I want to tell her, but they’re currently fighting about his job and I don’t want to add to it. What would you ladies do??