Another thing is - even if it is "just sex", her theoretical partner (the other girl) is more than a piece of flesh. It would have to depend on how SHE feels about being watched, because what you have described is essentially a threesome. Even if you are not touching either one of them, if you are voyeuristic, watching, turned on, playing with yourself, and/or having an orgasm because of their sexy-fun-time, it WOULD be a THREESOME because there would be THREE people involved.
If that is all good with all three of you (you, Eve, and whoever the other girl would be), then RAWK! It's these little stipulations and minutiae that are supposed to cushion one person's insecurities at the expense of others that send up red flags for me.
But I don't really care since I'm not one of the people involved. You do whatever makes sense to you.
Hope you have a good one!