There is zero shared ownership between old and new firm. Rather than buying out the prior owner, they just started their own company, working at same location as old firm, with prior owner retaining his personal office.
David - you're a cynic. Initial communication indicates they have not been paid by the other firm, having been formally terminated, and then hired by new firm a day apart. But I'm going to ask for clarification of that. Not sure what it would mean if they are providing unpaid services, but I'm thinking we wouldn't have to worry about that.
Nonetheless, not sure if nondiscrimination testing needs to include both sides.