The spouse must have an ITIN in order for the return to be e-filed. Otherwise, the return will need to be filed on paper.
The return can be delivered electronically to your client, so that he can print and file it according to the instructions you provide. They should also file the return using approved PDS so that his record of mailing may be recognized for US tax purposes.
No interest or penalty may be assessed if there is no tax due on the return.
P.S. As you may be aware, rental income is taxable to NRAs on a gross instead of net basis unless an election is made. And NRAs are subject to FIRPTA.
Still an AllStar