Helenaka78 - If your friends man has a problem with her using a vibrator on her own it means that he is insecure about his performance in the bedroom and he's taking it as a personal insult - that he isn't enough to satisfy her.
It could make him worried that she might go for sex elsewhere if he thinks he's not pleasing her. Your friend needs to reassure him, and also try and gradually bring the vibrator into the playtime she has with him, so he can learn to be comfortble with it. Once he is comfortable about them using it together he will be less threatened about her using it alone.
She could also try bringing in toys for the two of them to use together - http://www.lovehoney.co.uk/product.cfm?p=18919 or http://www.lovehoney.co.uk/product.cfm?p=22834