Don't sell cases without the HDD trays without telling the purchaser there are no HDD trays?
How many HDDs is he planning on using?
You could buy a couple of 5.25 to 3.5 bay converters - but it's not ideal.
Just the one HDD is all he wants to add in
I would ask if he's happy with the compromise - as a 5.5 to 3.5 converter wont cost a lot - but his upgrade path will obviously be compromised if he wants to add HDDs in the future.