Cross training and body awareness
Let me start this off by saying that I am not an expert in conditioning, balance work, range of motion exercises, rehabilitation, massage, chiropractic, or acupuncture. Probably you aren't either.
So my best advice is line up a support system for your dog, who is going to go through rigorous training, competition (potentially) and challenging physical exertion to play in the sport of agility. Get expert advice suitable for your dog (your friend's dog might be getting different advice for good reason).
Because we all tend to learn from friends who aren't necessarily expert, and you probably will adopt or at least try stretches and exercises, balance work and rear-end or core strengthening activities let me just say this: be cautious, a little bit of anything is probably fine but...
If you do an exercise enough to be beneficial to muscle development or strengthening - it may be enough to also over stress joints or soft tissue.
If you train your dog well enough to adopt un-natural postures, they may do them even though they don't feel good or actually feel bad.
And finally, sometimes doing less is more. If you are worried about a rear leg, working it more may be exactly the wrong thing to do, err on the side of resting rather than working.
For me, I prefer to let my dogs use their bodies the way they would like to, but challenging them all the same. So rather than balance work, we go for walks in uneven terrain. Rather than cavaletti, we go cross country over logs, heavy grass, up hills and down. Swimming and swimming against a current is another favorite. We play in snow when we get the chance, or run along the beach in sand and the edge of the water.
I think generally, the greater variety of games you play with your puppy, the better - but always supervise the puppy closely. Introducing them to something neat to do becomes their new favorite game and it only takes an instant of inattention and they go off and play without you - sometimes not safely.