http://www.irs.gov/businesses/small/international/article/0,,id=129631,00.html
Two ways to file taxes. 30% withheld for rent *or* file schedule E and claim expenses/depreciation. For the first year, the 30% is withheld, but if they file, they get the excess back.
To get the ITIN, as soon as the buy the house, they submit a copy of the contract with the w-7 form (if they buy it jointly, they need 2 forms). There is an exception under reason h that lets them get the number *before* they file.
Note, an issue that nobody ever brings up. If they die and it's worth $60K or more, you would have to file a US estate return. This can be a pain to do. If they merely grow old and sell, they have to deal with FIRPTA (10% of the sales price is withheld towards the final tax bill), filing a return and AMT. (When you sell, the schedule E depreciation is recognized, so they have to pay that back as well.)