Tell him you will attempt to T-cut it first (for your own piece of mind) yo ucertainly will not do any harm to the car for sure...
And a 10 year old car?
Is he having a laugh??
Is it a really posh car?
If it's an old Mondeo it probably isn't even worth £500!!!
Can you replicate the accident? Just to be sure the hose could actually have dragged along the car in the manner needed to scuff it?
But definitely make him let you try T-cutting it first, if it polishes out it's easier all round.
Also, what makes him think it was you that scratched the car??
If he tells you he saw the hose go across his car then fair enough...but don't let him get away with a re-spray
Ian