Ah, from an interest deductibility perspective, he should be careful regarding the way he plans on financing and purchasing the new property.
Since all the equity is in the house that will be rented, he will most likely obtain a mortgage on the second house which will be his principal residence. As such, the interest is not deductible.
He could structure his affairs to ensure the interest is deductible, but he would need to talk with a qualified tax practitioner. There are other tax matters to consider as well.
Todd Stokowski, CA