............ the crew do not have the discretion to waive the rules.
Nonsense; the Captain can waive pretty much any rule if the circumstances, in his/her judgement, require a particular course of action.
The problem in this case, as in so many others, is a poorly-trained FA with an attitude that tells him/her that their passengers are all idiots. You only have to read some of the stuff in the CC Forum to see what I mean.
In this case the FA should have immediately referred to the Captain who, I suspect, would have immediately let the woman try.
Maybe the phone wouldn't work, but at least she could spend the rest of her life knowing that she did her best, rather than being prevented by the stupidity of an FA, who ignored the SOP to pass the problem up to the Captain.