Through the use of the omniscient door, Mag could see all of the ailments that his customers were suffering from, but he didn't go around handing out diagnoses to everyone. After all, there were no perfectly healthy people in the world, and it was only normal to have a few niggles here and there.
Furthermore, he was a chef and a restaurant owner, so he really wasn't qualified to practice medicine. Just because he could diagnose everyone's conditions, it didn't mean he knew how to cure them.
However, if there were some regular customers with extremely severe conditions that had to be addressed as quickly as possible, then Mag would commit these customers and their corresponding conditions to memory. He didn't know how to cure them, but he would find chances in the future to issue a few gentle warnings to them. After all, it went against his character to remain silent despite having knowledge that could help others.
