My friends dos used to do that, she told me to take it as a compliment....ha! She stopped her pup from stealing by grabbing a toy of the dog, walking over to the dog adn as she would brab the stolen item she would repriand her, take it, hand her they toy and praise her. So its like stolen item =NO, toy = good
The fact that she is teethingmight enhance the need to get her mouth around more items but many people will tell you to puppy proof your home. Obviously that dosnt work when she is stealing guests things but keep a eye on her and try to catch her in the act before she runs off with it and exchange it for a toy.
Oh and I have a gf that thinks it is ok for her to reprimand my dog, it makes me soooo mad to. Its one thing if your dog is jumping all over them and they are saying get down but in any other case I agree, it is wrong. Then they get pissy if you tell tehm not to. Just say your trainer is trying to help you gain trust with your dog and at this critical stage maybe they can let you handle it! ha ha I can just see your lil thief now.....ooh new toys for me?