I had this same problem.
Obviously - mobility first, but something I find useful for getting the posterior chain working properly is Romanian Deadlifts from a low pulley (or Vogelpohl Deadlifts).
You can’t help but engage the glutes and arch the back properly with those.
Yea, when I played football I couldnt get into my offensive lineman stance. The trainer at my school told me it was my flexibility. I needed thigh ass and hip stretching 3x a day. Which I did for a week and my stance got alot better. I know a position stance is diffrent then deadlifting, but its still kind of similair. I think he should stretch every day.
Also how tall is your friend?
If he has a kyphotic spine you should probably have him doing some exercises to strengthen his upper back/shoulder girdle.
Theres a bunch of articles on here that will help:
Heal That Hunchback
Lower Trap Something or Other
…theres a bunch more. I guy I work with has it pretty bad. Its from sitting hunched over the computer all day at work.
Fixing his posture in general will do wonders for his deadlift progressions.