Of course, if she is capable of going to the event without having to rely on others, the mother is quite within her rights to make her own decision as to whether or not she attends the event. If it is case that she might simply get tired, then that's not really a problem. However, she cannot simply decide to go, if she needs to rely on others to care for her. It all depends on what her level of frailty is, and how much help she would need.
As an aside, my father was only 78 when he passed away last year. He had a sharp mind, was very independent, lived alone, and still managed to get out and about into the local town, even though he was riddled with arthritis, and had numerous other issues! Most outsiders would think that he should've attended a family event if invited. However, he was a constant worry to me, and everyone who cared about him. He thought he was invincible, and could do anything ... and he certainly did do whatever he wanted, to the detriment of my sanity, after frequent hospital visits due to falls ... some serious! But he wouldn't give up doing whatever he wanted to do, regardless of the fact that he was putting himself in danger! I loved him dearly, and went above and beyond the call of duty to do whatever I could for him. However, there came a point when I did feel that my life was no longer my own. Yes, he was capable of making his own decisions, but they weren't always the most sensible ones, and it was other people who had the worry, and burden, of picking up the pieces afterwards. I wouldn't have changed him for the world, but I have told my own ACs that I will never willingly put them through the stress, and worry, that my father put me through.