One of my travel companions can not stand for long. What do we do?

Bring a walker or a wheelchair, alternatively book for Fast Track / Easy On Board (priority boarding).

In most terminals, you can pre-book a priority boarding line, that will allow you to jump the queue. You can always ask a ground crew to help you jump the queue if need be. For example if you are on crutches or have a baby or toddlers.

In most cases, if one of the travel party battles with long-standing, bring a walker or ask the ship for a wheelchair - in many cases, just asking for a ship wheelchair can mean that the ground crew will speed you through the check-in process. 

Remember that if you need a wheelchair for your entire trip, you need to bring your own and you will have to keep it in your cabin. It cannot be left in the passageway.