If a person is on Soc.Sec.disability and becomes a distributor for a MLM company will his SS be cut?

The person will unlikely make enough to replace both the SS disability and the medicare/medicaid health insurance. He is avoiding becoming a distributor because he is afraid that if he makes $300-400 or more a month, that it will be subtracted from his other benefits or worse yet, he will lose his benefits because the government will think that he is capable of supporting himself. Is this true? If there is a limit to how much you can regularly make and still retain all benefits, this would be good to know. Becoming a distributor would likely bring in some money, but it could be quite hard to consistently bring in enough. The person in question only makes $12,000 a year on disability and is almost depleted of money from an inheritance. He really needs supplemental income. The disability is mental and he has not been able to hold down a job for any length of time for years.