If I remove shared content from a campaign

Let's say I've shared my PHB and a player in my campaign selects a background from PHB on his character sheet.

What happens to his sheet and the background if I stop sharing the PHB?

As far as i understand, the character sheet stays usable and fully functioning but as soon as any content that is no longer accessible is changed then it can then no longer be changed back.

However i am yet to test it myself so not 100% sure

Yes it is as Scotty said.

Thanks

What Scotty said is correct. You can also continue leveling up that character.