Griff, it is just so hard when parents, who were once so capable and fiercely independent, need support, guidance and protection, but they refuse to see it, and reject the help you offer. It is impossible to over-rule that decision by the very nature of the parent-child relationship, even though it becomes inverted as this thread title says.
In fact there is a Ukrainian saying which translates roughly as "If there are a lot of us, we'll manage to beat up Dad" which is about achieving the impossible ("Many hands make light work", sort of) and actually truly reflects how impossible it is for children to overturn a parental decision. At any stage in life. So please don't beat yourselves up, you and Pete. You did absolutely the best you could at the time with the information available to you at that point. x