While I hope he doesn't die I can see them scripting it in. Carol finds out about Nikki on the wedding day, has it out with David, dumps him because she doesn't trust he told her to back off. After slamming the car lot door and leaving, David collapses... found shortly after by Bianca who calls the ambulance.
Meanwhile Carol is having it out with Nikki. Nikki finally realises she's a b***h for trying to take David off her and confesses David did tell her to back off and she's been making it up. Carol slaps her, goes to find David, sees the ambulance and hysterical Bianca.
In hospital there is a tearjerker of a heart to heart, Carol saying sorry etc etc. David suddenly clutches her hand, has a heart attack. Carol screams that he can't leave her, not now, not while she needs him to fight the cancer. He whispers something to her and then he's gone. Carol then has to face up to the fact instead of her going, it's David.
Either that... or he just leaves the Square and leaves a note for Carol saying sorry having realised he really does have feelings for Nikki and this means he can't stay with Carol.