From the 2018 1099-r Instructions... excess contributions... losses 
However, taxpayers must include the total amount of the excess deferral (unadjusted for loss) in income in the year of deferral, and they may report a loss on the tax return for the year the corrective distribution is made.
Someone please correct if wrong... I would issue the 2018 1099-r for $900. But his tax preparer will include $1,000 as the overage, and will report a loss of $100, all in 2018.