I've had UA move me to AA 3 times recently. In each case, US messed up the transfer, and despite me checking in, I was not able to board a flight without the flight coupon attached, which United never gave me. In one case, I made a few "runs" between AA and UA terminals. In the last case (due to the issues in my previous two) I proactively asked AA what was wrong, knowing something would be, and found a great AA agent who spent lots of time with UA on the phone and got it figured out. It seems UA transfers the eticket to a paper ticket, but never gives me the paper ticket.
Anyway, my point is (should have gotten right to it, sorry) it's likely the AA boarding passes required something from UA that UA did not provide (the ticket) hence the denial of getting on AA. Glad Mom is safely with you.