The grammar behind 'above mentioned'

With regard to your friend's original statement, "below-mentioned" is a poor choice because "the issues" have not yet been mentioned. In other words, they are still pending statements...the below-as-yet-unmentioned issues? "The following issues" is more correct in both grammar and context. Or, quite simply, "the below issues".

Above-mentioned is indeed a single adjective or adjective phrase, whether it has found itself commonly constructed with hyphen, space, or neither. Unfortunately (for the sake of your argument), this is unlikely to have an impact on whether "below-mentioned" is a valid construction, since we happily make adjectives out of whatever parts of speech we want.

It's either above-mentioned (UK and older US usage) or abovementioned (US). As a compound premodifier, it should not have a space. Undermentioned also serves the same purpose as below-mentioned, although neither of them is particularly pleasant to the ear or the eye. Hereinafter can also be used (mentioned hereinafter).