No ... not legal. If he will not comply then you can simply put the deduction on your return with all his other info on a statement and attach it to the tax return and let the IRS work it out. If he needs to amend his return that is not your problem. Show him this from the IRS :
You must give the payer of interest income your correct taxpayer identification number; otherwise, you may be subject to a penalty and backup withholding.
https://www.irs.gov/taxtopics/tc403
