Hi Marla,
I might sit down with just the two of them to work this out. Recasting may not be a possibility, so they have to focus on the performance not their relationship status. Have they talked with each other about their relationship status? That might help resolve a lot of the awkwardness if they just haven't talked it out. Maybe they are worried that if they do talk it out that the other might talk about that conversation with others (we know how teens are), so an agreement that this is a conversation just between them, with you facilitating as needed.
I might then look at their objectives and tactics.
Is the Prince doing anything that is outside of the realm of the character while performing with the performer playing Delilah that is making her feel uncomfortable?
Is Delilah focusing on her own objectives and tactics?
A discussion of whether what intimacy is there is making either of them uncomfortable might be a good idea, just in case things have changed.
A final point of discussion with the two of them might also be to acknowledge the relationship friction, but that in the grand scheme of life they will move past this. In 2 months, 6 months, 1 year, etc. it won't seem so awkward. It might not help tremendously (especially for the guy playing the prince, because his feelings are unrequited), but I've had some luck with helping them put it in perspective like that.
Best of luck with this situation!
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Raymond Palasz
Auditorium Director/Director of Theatre
Munster High School
IN
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