Earlier quoted context omitted.
In your opinion, what are the practical risks to them? What actual interference in their daily lives has the FB leak caused or is likely to cause, which would justify e.g. the effort and inconvenience of switching their phone number? Has anyone abused the data leak against them in some way? If not, how likely is that someone would actually do so?
I guess identity theft is the most likely risk? but who knows? and humans are pretty bad at calculating such risks. Changing your phone number is a pretty concrete hassle though, and I guess none of them estimated it worth the hassle… It’s just some anecdata to support the parent’s point.
So from looking at that risk it seems quite clear that no, it's not worth the hassle, because the hassle is real but the benefits are not. It may be different for other risks, of course - e.g. if OP was worried about a specific abusive ex stalking one of their family, that would be a very different story.