How on earth would mutilating anything, especially a cat, make it easier to train? What a load of bs.
Declawing is cruel and unnecessary, and can cause a life-time of suffering for the kitty. It's a huge factor in the development of behaviour problems such as litter box avoidance and increased biting. Please don't take your friend's ridiculous advice. In fact it might be a good idea to show her some real info on the subject so that maybe she'll stop dispensing such nonsense.
Declawing: Why You Should Never Subject Your Cat to This Torturous Procedure
The Paw Project - FAQS about declawing