I did fail to reread the language before bringing it up. If he failed to turn a profit, however, it wouldn't be gainful, and employment generally means working for someone else in an employer-employee relationship. The idea being that if he were successful in his endeavor he wouldn't be coming back anyway, as stated in the next paragraph:
"Failure to comply with this provision shall result in the complete loss of seniority rights for the employees involved. Inability to work because of proven sickness or injury shall not result in the loss of seniority rights. The employee may make suitable arrange- ments for the continuation of health and welfare and pension pay- ments before the leave may be approved by either the Local Union or the Employer."
If he laid out his plan with UPS and the Union, got approval with the understanding that if his business was successful he'd not be returning, made arrangements for health and welfare and pension payments, the terms of the contract would be satisfied. The health and welfare and pension payment thing might make it unfeasible for him, and it may be unlikely that the company or union would approve the request, but it can't hurt to ask.