In response to your first question -
What you describe sounds like normal puppy play. You will KNOW if they are not playing!
Usually what to watch for if its play: Both dogs "swap over" between being the one that gets to be on top. Often the older dog will lie down and allow the pup to climb all over them.
Growling and yipping and so on is all normal and not necessarily a sign of a fight.
Often they will do a play bow, which is a signal to the other dog that "remember that whatever I do, I'm playing with you". Play bow is front legs out on the ground, bum in the air like this:
If you notice them doing any of the following, it may not be play:
Their bodies get stiff and tense, instead of "dancing" about and moving a lot.
They "stare" with a tense closed mouth.
They snarl with a still, tense body, maybe with some whites of the eyes showing.
Whether you should get involved when play gets rough is a difficult question.
My opinion:
You have the right to stop noisy and annoying play whenever you like. Both dogs must go lie down and get quiet.
It is more complicated if they are actually fighting. Then you will have to look at the bigger picture - what is causing the fighting to happen.
In the situation with your friend, I think your idea to get a trainer in is best.
