Let’s try to keep the discussion civil and constructive guys. This individual came here looking for help/advice; both of you started out giving him just that (based on your own personal experiences), why not return to that goal.
Let’s also remember that everyone is an individual and will have different strengths, weaknesses, preferences, and limitations. We really know nothing about this guy’s brother in law other than he’s 15, wants to start BJJ, and hasn’t previously been involved in athletics. He could be really naturally flexible but weak, really naturally strong but stiff, naturally explosive but with poor natural endurance, really naturally good at endurance but have poor explosiveness, etc…,etc…etc…
As a result what specific strength and conditioning exercises will best help this individual in their BJJ training may vary and may be different from what would be best for any of us. In the end all that really matters is results. If squatting and dead lifting helped Magick improve, then how can anyone say that they were the wrong thing for him to do? If lots of flexibility training and avoiding squatting and dead lifting improved Jarvan’s BJJ, then how can you argue that this was the wrong path for him to follow? Even among the elite in the BJJ world you have individuals with very different “styles” based on their own individual physicality. For instance, Eddie Bravo’s style is all about flexibility and doing tons of DL’ing and squats to strengthen his hips would likely decrease his mobility and actually make him less effective as a grappler. On the other hand Ronaldo Souza’s style is much more based on power and explosiveness and focusing extensively on flexibility at the expense of strength and power training may have decreased his effectiveness as well. Both men were World champions, so can you really say that one was “right” and the other was “wrong”?
The exercises that I mentioned are however integral movement patterns in BJJ/submission grappling, apply to everyone, and regardless of individual physicality, are going to help a person improve in their grappling pursuits. In other words they are specific grappling strength and conditioning exercises as opposed to non specific GPP types of exercises (like squats, DL, chins, strongman stuff, battling ropes, sledge hammer drills, gymnastics strength elements, Oly lifts, etc…) and so do not require that we individualize them to him specifically. If he wants to add additional load to the movements (like a sandbag or grappling dummy) once he is sure that he has the movements down correctly in order to develop greater strength, then of course that can be beneficial.
Once he actually starts doing BJJ, then hopefully his coach will help him to individualize his strength and conditioning program to help him improve best based on his strengths and weaknesses.