Although the deadlift is a fantastic strength-building exercise, doing it incorrectly can be dangerous. When you're watching your partner lift, it's your job to keep her safe. The upper back can round, but if the lower back starts to round, then there's a problem. Remind your partner to push his hips forward and push his feet into the floor. It's also important that you help your partner choose weight that's not too heavy. He should be able to perform all 8 reps perfectly.
Perform all sets and reps of the split squat for one leg before switching to the other; the same goes for the reverse lunge. Each leg is going to be screaming for rest before you finish the set—don't quit early!