How long is the purlin? I have no SE basis for saying it needs supporting, but I have done god knows how many loft conversions and can't remember any purlins over more than about 3m that haven't been supported mid span, what size and length are the roof rafters. Just because your man hasn't taken them out doesn't mean they shouldn't be there.
Purlins usually indicate roof rafters that would otherwise be insufficient to span the whole length of the roof and themselves unless of short dimension need a support of their own to prevent sagging.
As our SE friends on here have said it's unlikely to completely fail, in fact I know of a fink trussed roof that had all the compression and tension cords cut out and held up for 15 plus years, but that doesn't mean it's right.
Quite clearly this isn't a "posh" conversion but a "posh" boarding out and I have seen much worse done, but will never be able to be classed as a habitable room.
Just a thought but but the OP mentioned a binder that had been bolted to the 9x3 was the same thickness but an inch smaller 8x3?), doesn't sound like any binder I have ever seen.