A book that I just finished reading deals a bit with this issue. It's called You're Not You by Michelle Wildgen.
"Wildgen's first novel centers on Bec, a self-absorbed college student drifting through school and an affair with a married poetry professor, and it shows real promise. When Bec takes a summer job caring for Kate, a young married woman with Lou Gehrig's disease, it seems easy to spot the formula: lost soul comes of age through the wisdom and resolve of the terminally ill. Where Bec is anxious and aimless, Kate is sarcastic and at peace; despite paralysis, she teaches Bec to cook extravagant meals, fund-raises for ALS research and spouts wicked one-liners. But when Kate kicks out her cheating husband, Evan, Wildgen's writing becomes clear and determined, daring to spotlight an almost taboo subject—the need for sex among the sick. As Bec takes on more of Evan's roles, eventually moving into Kate's house, Bec's deep and conflicted feelings for her charge allow Wildgen to navigate the complicated moral territory of Evan's, or any young spouse's, responsibility to his terminally ill partner."