Your friend can buy as soon as he wants, if he pays cash. If he wants to qualify for a mortgage, he'll have to wait at least two years for a VA loan, 3 years for an FHA loan, or 4 years (if he's putting at least 10% down) thru Freddie Mac.
Of course, he'd need to re-establish good credit after the foreclosure, and pay his bills on time.
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Senior Mortgage Consultant
there is also the possibility he can find owner financing on a property, but he can safely assume any loan is gong to cost more and the longer he can re-build his credit, the better.