No it was a small carrier with few partners. And the family member didn't even have that boarding pass in hand yet. But even if there had been no connecting flight at all, the pass should have been honored.
Denying lounge access to someone who just walked off an 11-hour United flight (not to mention the preceding feeder United flight) is a customer-hostile policy.
The LHR UA lounge is a UA club. The UA club entry policy is that it is a departure lounge and not available to arriving passengers who are not club members. Had the person been departing on a *A flight they likely would have been all
wed in. I'm not saying I disagree with you -- just what the policy is.