You can call UA to have a note put into each others PNR's, but thats about it.

When I have been traveling with other in mixed classes, I have always been able to check us in at the same time.

As far as visiting Grandma in C, you will get several different opinions here. But the norm seems to be that is is tougher to visit someone in C if they are flying Y (there is no issue with going from Y to C, its "visiting up" that is discouraged).

However, I would speak to the FA's and have the companion explain the situation, they may allow the companion to check on her a few times during the flight (so long as she is NOT a distraction to the other passengers... ie: loud talking, getting in the way during meal service, etc.).

YMMV.

You don't ask... but maybe see if the companion can purchase a upgrade for cash the day of check in? That would totally solve the problem.