There is no UA lounge at LHR and would need to be able to access a Star Alliance lounge but I think you need to be departing in Business to do so.

staralliance.com/en/…

Thanks for the quick response. I guessed that was the case, but thought I'd ask.

If its a long layover you could always try buying a day pass to a lounge.

Yes as long as you are a departing in international business class.

OP is departing with a coach ticket. No free access.

If they're flying Star Alliance onwards it's quite possible they'll have access if it's a same day connection from business class. I'm fairly certain all the alliances now permit this to stay competitive with one another. The Star Alliance lounge is pretty central to Terminal 1 so it's worth an ask, provide the onward BP and inbound BP stub and see.