It sounds like your girl took the fake root of camming, and I mean no offense to that. It's just that if she is truly acting the whole time, yeah, she's not going to enjoy it. I always pity girls like that, but I kind of am out of pity lately.
Do you know you much it would suck to shove things up you all day with no pleasure at all?
Ugh.
It might ruin her sex life off work too, which can bother you I'm sure. Having stuff up her might become more of a work thing.
I'd sit her down and talk to her dude. Don't try to get her to quit of course because you can't do that, but still...
Why shouldn't she cum? You do know that if she orgasms, she'll get tighter, right? Wouldn't you want your girlfriend to be in pleasure, or would you rather deny her that?
I don't see the point in it, really. If you force her to fake her orgasms, she'll start faking them in bed too. She'll lose connection with that part of her body.
Orgasms are awesome, yeah, but they're not something you can -only- have when you're with your partner. That's like telling guys it's cheating to jerk off.
Besides, if you encourage her to relax and enjoy the masturbation, or hell, even bring toys into bed to help her associate things as pleasure instead of "just work," she'd be so much happier. Don't you want her to be happy?