Given that it is you seeking direct advice and not your friend I would lead him into this slowly, too much too quickly and he may bail on you. You say he is motivated but if you push him too far too quickly he may quit.
To keep him motivated I would suggest taking his weight and measurements so that depending on what is the best lost each week you can use that as his motivational prod ie wow geezer you lost 4lbs OR you the man your waist is half an inch smaller.
They say that it takes up to two weeks to habit form so I would start him for the first two weeks with
2 full body routines and then 2 hiit sessions at convenient times when you can join him for motivation. I know this doesn’t seem much but with his size he will lose wieght
I would also clean up his diet but don’t restrict him too much to start.
For example if you get him to have chicken etc let him have a sauce on top so that he likes it, it is still way better than kentucky fried chicken. And let him have a ‘proper’ cheat meal once a week.
I assure you as he starts to get results you can up the number of workouts and the tempo, and improve his diet further.
Lastly if you are using a gym, if he is conscious about his size try and start by visiting when it’s a little quieter to build up his confidence…nothing worse than feeling fat whilst benching with some muscle dude peering over waiting for his turn.