I'm a person whose average sitting hours are 8-9 hours so I believe I've a weak back. If I sit on the floor, why do I feel comfortable to round my back rather than sit with my back straight. If I try to sit with my back straight without over arching my back, I quickly tire out and end up rounding my back again. Is this bad?
When your back is arched forward the load of your body weight lays on the passive tissue and bones while when your back is straight it requires some balancing act from your muscles. That's why we tend to slouch when sitting for a long time because our body tends to go for a less energy-consuming posture.
Although it's not harmful when it happens for short periods and with small loads like our own body weight, when sitting like that for a long time or under heavy load it is possible to cause damage to our passive tissue, bones, and such.
Of course, photos of the case described would help to give a more specific answer.