My experience, under audit, has been that the IRS does not want you lumping unrelated "businesses" together. For instance, they didn't want snow plowing combined with construction. Your client must have some method as to how he arrived at the total income for each endeavor. The burden of proof is on him. We all take in cash payments. Reporting them is on our honor. I would put the roofing and the music on separate Sch Cs. The waiting tables should produce a W2 or I would think that his employer is being remiss. Have him dig, dig, for deductions for the roofing and the music. He should be able to construct miles for both jobs if he kept any kind of records as to where he was and when. Not surprisingly, there are many clients these days who are trying to get by in this manner. I have an artist who also plays in a band and works parttime at a Hardware store., etc.
Other than the construction worker with the snow plow, I have never had any issues with any of these. Have him be as honest as he can. My audit issue kept terrible records, but by the time we got through with the second IRS meeting, his memory came alive and he produced a pile of receipts and a decent mileage log. This one also involved children and EIC. Therefore, I would do the best I could with your friend and not worry about it too much.