I'm guessing your mom does not have status, an economy ticket bought on United, and was upgraded with United miles or something like that? This question might be best asked on United if that's the case.
There is a United Lounge at Narita as well, but it's currently closed. It might be open depending on when she travels and there might be a reciprocal deal with ANA for United customers. She might have access to the United lounge or to ANA through that. Best asked on United.
The ANA board might be helpful if this ticket was an ANA ticket. A business ticket that was bought, but no Business seats on the NRT-HAN flight should give access to the lounge, but I'm not sure.