The job which this brother was performing is the kind in which halal and haram have been mixed together. At the same time that he was serving food that contained haram ingredients such as pork or lahm ul-maita (meat not properly slaughtered), he was serving things that are halal as well washing dishes and glasses. If he had known at the time he was working about the prohibition of the haram things, then his income in this situation comprises mixed money, both halal and haram. So if he wants to repent to Allah, then he needs to get rid of the portion of his income which his haram, by estimating and approximating to his best judgment how much the haram portion of the income was which was earned via haram service. He can spend it in any of the various charitable means, and he cannot benefit from it whether by investment or otherwise. As for the remaining portion which is the halal amount, he is allowed to use it to his benefit.