So he's been together with her for 10 years, he meets someone new, and he makes his long-term partner change so she can be more like the new one? I'd be frustrated with her, but only for listening to him rather than stand up for herself.
If they've been together for 10 years, he must not find her unbearable when she's herself.
Right now, it's looking like he's only dating you so he can show her an example of what he wants her to become. And then, he might even stop things with you since he's got the changes he wanted. He really sounds like a douche.
I'd have a conversation with him about how it's inappropriate and disrespectful of him (towards both of you) to act that way.
Then I would probably try to spend time with her and help her find her own style, rather than merely copy you.