What have they tried other than many parent talks and what did those talks achieve? More importantly was there any listening to find out what the problems are and letting them make decisions during the "talks".
The advice from ds(15)'s school for his year (S3, so no exams until next year) is they study for 1-1.5hrs each night. His school gives out very little homework so he needs to study at night to keep on top of his subjects. We haven't had many "parent talks" as he has no idea what his plans are after school so doesn't have something to aim for yet and thinks its all a waste of time as when will he use simplifying fractions after school and I cant really argue with that. Talks (usually casually and regularly just the two of us in Pizza Hut) consist mainly of, you are in school anyway so you might as well do your best as the better results you get the more options you have when you want to make decisions later and how can I support you revise the way that suits you best.
He wanted, and we managed to give him a clear office desk/space away from distractions (doubles up for me to WFH), places to keep his books/stationary etc, he said reading from his school notes was boring so we bought some revision guides, showed him BBC Bitesize, told him to explore YouTube etc. We talked about what the school expected (1-1.5hrs) and what he want to do (obviously bare minimum of 1hr!) asked him if he thought it was best to revise straight after school or after dinner etc night? Talked about organisation hints and tips, keeping track of which subjects he's revised/still has to do/to note things he finds harder so he can add to the list before he forgets. What to do/where to find help if he finds something hard - for example he knows he finds open ended chemistry questions tough so we have searched for examples on line/in past papers to try.
Obviously he'd rather be doing something else but he is now in a routine/habit that it needs to be done after school/dinner or activity and before chilling out on PS4/screens. It is showing in his class assessments which helps motivate him too.
It can all be very overwhelming being told to study, but not knowing where to start. His mum needs to open up a two way conversation on the subject (in Pizzahut!), listen, come up with solutions together then support him daily following through, watching out for when he is overwhelmed and needs a night off.
It is difficult when they are this age, the mum really should take some ownership of the problems and post directly here herself if she wants support rather than relinquishing responsibility by asking you to find out how to help your friends, mum help her son to study.