Anecdotal. A lot of managers have said that foreign players have brought with them a healthier approach to life and have tempered the drinking culture that was/is so prevalent in British culture.
No problem at all. He's not going to be near the first team next season so sure he'll have plenty of time to get back into shape.
To use a cycling analogy, coz that's what I know, Bradley Wiggins would regularly go on benders outside of competitions as a youngster. He'd won Olympic medals and all, so surely he deserved it, but he used to finish outside the top 100 in the Tour de France in those days. Only when he stopped pissing about and started taking his sport seriously did he truly make the most of his gifts and start challenging for real honours.
Duffy is on easy street, earning a salary most of his peers would die for. He could easily swan along, playing a few games each season and pocket the cash, or he could dedicate himself to his profession and make the most of his talents.
Fairly sure for instance that both Ronaldo and Messi are tee-total. Not that they have anything to teach the lad.
Shane Duffy isn't injecting stuff in between comps to improve his performance though, unlike Wiggins.