Even though I haven't run into a situation like that before due to my own reasons, I think that I'd be fine with that. I think it's normal for girls and guys to do that kind of stuff as long as it doesn't interfer with the sexual life the two of you have. What I'm trying ot say is say you aren't available and she's horny, she should be able to masturbate, but if she does it because she finds it better than the sex you guys have, there's something wrong. Since from reading what you said, this doesn't seem like the case so I think it's fine. Maybe you could even play upon that and ask her to do that right infront of you
THink about how aroused both of you will be afterwards and the kind of sex you'll have...
My 2 cents, I hope they were good enough cuz I don't have anymore spare change left lying around lol