Any unconfessed mortal sin is by definition damning to hell. However, given the confused state of the Church after Vatican II it is possible that Catholics who disobey the Friday discipline do it out of ignorance, rather than out of wilful disobedience. If that is the case, there is no mortal sin.
if I were trying to prove this was so to a Catholic, what would I reference?
Or maybe I’d better not - if they are violating this out of ignorance, and then I tell them, and then they scoff and don’t obey, well......