Or you could say, "He looks good in a suit."
Sorry. To be serious, I would ask the mom, "Exactly what do you want me to say?" Then shoot her down, "I can't say that, he's not a good student. I can't say that he has great drive and ambition, he doesn't want to do this, he's just giving in to you. I know of nothing positive to say about him that would help him to get him into college."
Are these people cousins-in-law? I think I would refuse my own sister if she was pressuring me to write a letter that was not warranted.