To be honest I really don't see a problem here. Even if it's not just an urge and she does it fully consciously, just think of it as a sort of masturbation.

In my experience with relationships, granted not overly huge, I never had an issue with masturbation with my partner at the time (for both of us). So if your case is the same and in your relationship masturbation isn't considered a form of cheating, neither should be lucid dream sex. You might argue that lucid dream sex is much more realistic than masturbation - well, yes, sometimes. But other times it can be too short, or too foggy or whatever. Even if it's one of these times that feels better than real life sex, some people are remarkably good in visualization and imagination and can visualize nearly as much details in waking life (while masturbating).

Honestly, if you ask me, dream sex should be considered "less of a cheating" than masturbation to porn. Doing it without porn means that you essentially get all of the fantasy from your own creative mind. I mean, you do understand that the person she had sex with in the dream wasn't real - it's just a simulation completely originating in her brain. So by this logic watching porn is more of a cheating since it involves other people - not just the ones that the brain simulates.

To sum up my 2 cents, if masturbation is something that is "allowed" in your relationship, I see no reason for lucid dream sex to be an issue. If it's not something that is allowed, then you can make an argument to your partner to out-law lucid dream sex too (though to me people that insist on banning masturbation or lucid dream sex in a relationship are plain illogical. But fine, to each his own, I'm not gonna force my logic on anyone).