Are there new leaves showing up on the "sick" one? If so, ther's probably nothing to worry about. Hibiscus leaves last about 8 months, then die. Sometimes it seems to happen all at once and is a bit alarming! You could move that one into a spot with less sun until it recovers - that seems to help sometimes.
If it's not producing new leaves, I'd check the soil for grubs and the plant itself for signs of disease. I'm not familiar with this problem, so I attached a link to a ste that does a good job of helping identify and treat problems.
Barb

Good luck!
Barb

Here is a link that might be useful: Hibiscus disease help