There is no reason who anyone could die from over sex.
But most likely those men are not very gentle and occasionally just kill those slaves.

I woud expect that the reason of death may be that those guys get bored and just kill that woman to get new replacement.

I have doubt about 50 times a day number if that is everyday average and not just maximum, But if this is true she can die of lack of sleep and malnutrition because there will be no time to rest and eat.

Only way to die from maddens is by committing suicide.
but if you get into strong depression your health also gets worse and you become susceptible to infections.