'except for one person' is a correct and usable phrase in written English.
You can use it to indicate that everyone is doing something, apart from a single exception. For example, "Everyone in the office was wearing a mask, except for one person."
He tested the desks with a group of more than three hundred volunteers at the Mayo Clinic, all of whom liked them, Levine says, except for one person, but he didn't like working on a computer anyway.
To play "Hunt the Thimble," have everyone leave the room except for one person.
They're all lions and they're all hungry except for one person, who's this guy at the end whose name is?
All were treated and released except for one person, who was in fair condition, a nursing supervisor said.
The position had just been created so she would be managing a team of existing staff, and everyone welcomed her except for one person, Terry*.
I haven't started naming names except for one person, and that's Mr. Richard Cheney, who I thought did a great job the other night.
That first place was a wooden room, no windows except a light bulb and just space enough for one person.
As my UMKC colleague Bill Black has noted, not a single major player has been indicted in the recent financial scandals -- except for the one person who walked into a police station with his hands up and surrendered (Bernie Madoff).
Returning to the landing area, I noticed I was alone on the dock except for one other person, an older woman looking out over the Hudson.
They walked past men and women sitting on the bright blue-and-beige carpet to an area almost empty except for one other person, with more than 30 folded chairs set out for guests.
