Hi Ali
Searched scotlandspeople who have all the Scottish records (free to search but pay to view records if you've not seen it).
No birth coming up for Samuel Edward/Edwards 1770 - 1800 Renfrewshire - or in fact for the whole of Scotland!
Nothing on famsearch either.
Not to say he wasn't right - they could have been non-conformists, or that part of the register is missing etc, but looks more unlikely.
No marriage John Edward/Edwards Renfrewshire 1770 - 1790.
2 marriages John Edwards and Elizabeth 1770 - 1790, one marriage between 1780 and 1785 for the whole of Scotland. (You could narrow this down by the trying each county singly ......take a little time.....)
Good luck!