Absolutely use the info provided online to prepare, including calling the number provided to be sure you have everything you need arranged. And on the days of travel, allow a little extra time and be sure to check with counter/gate agents to confirm arrangements are in order as requested. We've been most pleased with the service provided through UA.
And if grandma is reluctant to travel with a bulky walker, think about that we have used several times with great result. It squeezes to fit down plane aisles and folds up like an umbrella to go in an overhead bin. FA's have just raved about it. Also, if she cane get by using a cane consider that as an alternative/in addition to the walker.