it's 45 minutes of walking. he isn't even "that" heavy either as in there are people far heavier who have done nothing and did 45 minutes of jogging and had zero knee issues.
i think he likely has knee issues possibly because of incorrect posture, gait, form, shoes. or because of a joint issue caused by incorrect movement or positioning day to day.
i'd say do some leg exercises to strengthen the surrounding muscles. that means weights and squats. it has to be with correct form. focus on form over everything else. incorrect form will make the issue worse. stretching before and after will help too especially with age.
however knees shouldn't really hurt from walking especially on a treadmill. twisting and turning like when playing football is usually what causes pain as well as impact. he is walking not jogging or running so impact is less of an issue especially on a bouncy treadmill.
so i don't see his issues being caused by impact as he's walking on a bouncy treadmill. he isn't twisting or turning either. it's likely an issue with his knees or supporting muscles / joints.