Any one of these should help:
Single Leg Calf Raises
Barbell Standing Leg Calf Raise
Calf Press
Standing Calf Raise (be careful with this one)
Doing calf execises stretches your Achillies tendon, so don't forget to
always stretch/warm-up properly before starting any weight training lest you pull/tear something.
Squats are also a great exercise for your legs, as a whole.
Also, make a bet with your friend. If one of you misses a workout session, they have to pay the other a sizeable amount of money. Its good you workout with a friend because it makes your workout something to be enjoyable, meaning you're more likely to do it again.
Hope that helps. Goodluck.