Candi is correct.
Your friend is not the leader of his dog. The dog considers anything he steals is HIS ITEM and how dare the owner try to take them away.
Here is how Ed says you can solve the problem. (But I would first do the Ground Rules with this dog as the dog thinks it is the alpha). I realise in this case it's not his toys the dog is attacking over, but the concept is the same.
This is copied directly from the link Candi supplied, I've edited it for length.
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Your Toys
As puppies grow up they should not be allowed to become possessive of their toys. Your attitude needs to be ALL TOYS ARE YOUR TOYS and you allow the pup to play with YOUR TOY.
The handler should be able take every toy away from the dog at any time without the dog showing aggression. If the pup growls, there is a protocol that should be followed. I have covered this in my DVD on Dominant Dogs.
If the dog is older and there is a concern over being bitten, a leash should be put on the dog and the dog should be corrected off the toy. If there is any growling at this point the handler should scold the dog with a firm, deep voice.
Once the dog has dropped the toy the handler should lead the dog away from it before bending to pick it up. Until the dominant situation is under control that toy is never given to the dog again. The handler controls his dog’s universe and the dog needs to learn this. The factor in that scenario is, "You determine what toys are played with and for how long."
If the dog will not give up a toy when corrected on leash and the handler does not feel comfortable with his ability to safely get the toy out of the dog’s mouth, there are 3 options available:
1. Ignore the situation, only pick a fight that you can win. When the dog finally releases the toy, pick it up and put it away for good. He doesn't play with it anymore.
2. Trade the dog for a piece of food. When he drops the toy put it away and set up a training session where you are in a better position to correct the dog for not releasing the toy. (possibly having a muzzle and prong collar on to make sure you will not get bit)
3. To win the toy war with a strong mature dog takes planning. If it's done wrong you will be bit.
First put 2 collars on the dog. A Leerburg Dominant Dog Collar is the best to first put on the dog. (Click here or see photo below) The second collar can be a normal collar.
Attach a leash to a post or a tree or solid object in your home (i,e, around a door knob and then close the door on the leash so the long end of the leash in a different room than where the line is over the knob.)
When the dog has the toy in his mouth, put him on leash and take him to the location of the second leash that is attached to the post. Clip the second leash to the dominant dog collar on the dog. So the dog now has two leashes on, each attached to a different collar.
Now tell the dog to drop the toy and back up and stretch the dog between the post and you. Put enough pressure on him that he spits the toy out. As soon as the dog spits the toy out verbally praise him. If you can approach without getting bit, do so and calmly praise the dog. If he tries to pick up the toy when you approach, don't get mad and scream. Just calmly say NO DROP IT and back away as you tighten the leash again.
This process should not be a wild and hectic operation. Keep your cool and remain calm. By remaining calm you greatly increase your image in the dog's eyes - he sees that you can choke him until he passes out without getting mad. This procedure does not hurt the dog. There is no real pain, it just takes the air away from him and he submits. If the owner makes a fight out of it and gets crazy and screams it defeats the purpose of what you are doing.
Continue to choke the dog as long as he goes after the toy when you approach. If you don't feel comfortable picking the toy up, then kick it out of his reach. This is one way that you only pick a fight that you can win.
This two leash process is only necessary with big dogs. Little dogs still need the dominant dog collar but they are simply picked up off the ground and hung until they drop their toy or stop acting aggressive.
Some handlers are simply too soft in temperament and they will not do this. My position is that when the option is to solve the problem before the dog bites someone - or to do this rather than put the dog to sleep - well it's a simple decision for me. I would rather deal with handler aggression with our dominant dog collar a few times than by taking the dog to the vet and killing it. In most (not all) cases the problems have been caused by the owners not doing their job properly. So here is part of the fix on tough hard dogs (toughness and hardness are not a size issue - small dogs can be very hard dogs).
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