Sexual behavior must be consensual to be moral. Consent requires two things. A statement of affirmation, and a comprehension of that which is being consented to. I hold that children do not have the emotional or intellectual capacity necessary to comprehend the consequences of consent to sexual behavior. As such, children may not consent to sexual behavior.
In general, I've been content to recognize the standard measure (chronological age) of consent, knowing full well that it was imperfect. Not all people reach a mature enough state to transact their own rights by 18, and some reach this level of maturity earlier. It's hard to say what the right answer to this situation is without talking to the 15 year old. Even then it's a bit dicey... but staring death in the face makes you grow up pretty damned quick.
A close friend of mine in high school was dying of bone cancer.
His Pops hired a nurse with 'special' skills to help attend to him .
He died before he turned 18.
I think it helped make the last couple months more tolerable.