I believe the property stays intact and as long as he pays the taxes and HOA fees he should still be able to collect rental revenue to any US bank account which he could then have wired to his home bank in whatever country he resides. If you are acting as property manager you would be required to hold up to 28% of earnings in an escrow account to cover taxes in the case your client has no SSN. If yes then make sure you have a W9 completed and on file.
Your client should have a contingency plan so that, if deported, a trusted individual can act as property manager if property becomes rental or have power of attorney if client decides to sell, must continue to pay property taxes and mortgage loan to avoid foreclosure.
You do not have to be a us citizen to purchase property. He will still be the owner as long as he is making monthly mortgage payments or if its paid for.,
I have never had a client 'deported'; however, I have clients that are not citizens that own property here. They were here for work, were transferred back overseas and decided to keep the property to rent. If he sells he will need to follow IRS code Section 1445 where taxes will be withheld at closing from the proceeds. This is also referenced in item 20 in the 1-4 Family Residential Contract.