Yuck - what a mess. Your fiance getting paid cash under the table was wrong. The boss should have reported it to the IRS and withheld taxes, and your fiance should have reported his earnings to SSI.
Whether or not this affects his SSI and his food stamps will depend on how much he earns. What he should do (today, or Monday, and not a day later) is to call his social worker who approved his food stamps and let the worker know he is hoping to get a job roofing, and how much can he earn and still keep his food stamps.
As far as SSI goes, it depends on why he is on SSI. If he is on it due to a disability, I believe he can earn about $600 a month and not lose his benefits, but he needs to check the social security website (
www.ssa.gov) to see the rules, or call up their 800# and tell a worker there he is considering going to work and can he do so and still collect his SSI.
Both the boss and your fiance were wrong on this one. If you are lucky, he only earned a small amount before now, so he's not likely to get caught, and will just pay taxes on what is paid on a legitimate W-2 and pay stub from now forward.