How long has it been since the surgery? I would get input from his doctor and physical therapist. After the first few months lunges and step ups (then step downs) are added in, but I would not do anything without clear guidelines from his md and pt. Usually after about 4 months most are advised they can resume normal exercises and activity.
as Janet mentioned, within the post-rehab phase, the physician and PT should direct the level of activity. After that, there are few limitations except those very jarring to the joint. People with knee replacements are also advised not to kneel.
As Karin & Janet pointed out, you definitely want to get the input of the surgeon & the PT. I train a 71 year old female who had one knee done just over a year ago – she feels good but still is not comfortable with certain movements so I just work around that. Get the medical feedback & then simply rely on your client for their input as you try adding back certain exercises & you should be good to go. Good luck Patrick & I wish your client a quick recovery.
It’s super to keep the doctor in the loop. Each person I’ve trained with knee replacements have been different. As you probably know, it’s important to communicate and ease into exercises through micro-progressions. Talk with them about what feels good to them. One of my current clients with a knee replacement can do just about everything. When he kneels on that knee it is numb. Interesting!