Yeah, how do you tell a friend who has invested lots of time and money in a restaurant that his/her food isn’t good? It may just be your personal taste and if they are staying busy and getting good writeups, keep your mouth shut. Surely you can find something there that is palatable.
You might try suggesting through casual conversation that you tried some ribs somewhere a different way, and that you thought they were good, then ask him if he knows how they may have done it….might get his curiosity up. Cooks usually like to learn others methods, tricks. Any business owner that doesn’t try to learn and grow (change) is bound to failure.
Go with him to another Q place to "spy" on the competition. After eating the comp food, make comments about this being good or that being how you prefer it. Maybe he’ll get the hint.