Section 221 says that the deduction belongs to the person who paid it.  A guarantor who made some of the payments can deduct that share of the interest; a "default" is not required.  

An interesting point here is that the $2,500 annual limit on the deduction is not per loan, it is per taxpayer.  So if the total interest paid is $5,000 and the parents actually paid half of it, they can claim $2,500 and the student can claim the other $2,500.  

"It is what it is" is the phrase I use when someone asks me to define "vacuous," but that's neither here nor there.