I’m going to go out on a limb and make a few assumptions. OP has posted a few questions in the Hawaii forum, so I’m going to guess that’s the destination and pax is either flying yyz-yvr/yyc-Hawaii, or yyz-usa(mainland)-hawaii.

If via USA it is possible that flight ex-USA is a codeshare operated by UA. In such a case guest passes won’t work.

If on AC metal the OP likely booked Basic or Standard and therefore can’t buy lounge access on MMB.