I recently lost over 30 lbs and am on my way to lose about 15-20 more by continuing alternating cardio and weight training.

My brother wants to lose about 30-35lbs and has recently joined a gym - but, in addition to calorie deficit, he only plans on swimming (alot of laps) and treadmill everyday - he has been doing this for about 2 weeks now with one day off a week. Shouldn't he change it up a little, or is swimming considered a form of weight training and treadmill cardio? Shouldn't he have more rest?

Hey, if it works for him; great!!! Weight is an "aid" to help with the weight lost. It helps a lot but not necessary to get result. Swimming but threadmill is awesome, that is both upper and lower body stuff; especially if he does butterflies!!! If he wants to try, do some HITTs 2-3x a week.

He doesn't have to change up the cardio. Whatever works for him is great, although he should definitely give himself rest in between. But weight training is a great idea for two big reasons: 1) it will help him gain/retain more muscle when he is losing weight, and 2) added muscle is the best fat burner out there

Building muscle will make you look better and become physically stronger. So both are necessary imo.