Going for the kill...

Friend has a four month old puppy...training coming along nicely except he's very bad in the park with other dogs...he only has to hear a faint barking in the distance and he starts barking himself with no stopping...also if he see another dog he pulls on the harness and get really aggressive, barking and generally making a right nuisance of himself...he's only a small dog...yorkie size but thinks he's huge I'm sure, takes all of her strength to keep him back. Whats the best course of action. He hasn't had any bad experiences with other dogs so I'm not sure why he does it. I've never had any probs like this so don't really know how to advise her as the best way to go about training him. All help greatly appreciated. Thanks

Has he been socialised well with other dogs?
If she can find someone with a calm dog maybe leave him on the lead but introduce them and walk them together in the hopes he learns not to get aggressive.Is she sure its aggression or maybe he wants to play.
My previous dog Max was attacked twice as a pup and displayed these symptons on lead,if you allowed him to meet the other dog and sniff noses he was fine,if he met another dog and he was off lead he was fine also.
In his case it was fear until he met the dog and knew he was safe.

At this age things can be improved a lot, and, to save possible problems for the rest of his life, I'd actually invest in a very good trainer or behaviourist.

Try www.apbc.org.uk or www.apdt.co.uk (The latter are mostly trainers and vary in experience).

It may be that the pup is just frustrated/excited, but OTOH you don't want him to learn to scare other dogs away or anything...