Not so. If OP's mom boarded before the UG cleared, it was absolutely the GA's discretion as to whether to go onboard, locate her and have her swim upstream. All depends on the GA, what other tasks there are and how the boarding process is going.

If the choice is delaying the push vs. finding a pax who has forfeited the UG by boarding, that's an easy one for the GA (and the other pax).

I realize that this is awkward when the recipient is not a regular flier as a GPU-user would ordinarily be, but the only way to assure that this works is to ask the GA for direction. Some will guarantee that they will come find you. Others will tell you to wait at the gate and others may tell you that you may board and that they will do their best to come find you, but without any guarantees.