you'd have to ask the PARENT LOAN lender whether they would let you cosign. kind of doubt it but it doesn't hurt to ask. you would of course have to make the payments on the loan to get a student loan interest deduction.
as for a personal loan here are the requirements for the loan to qualify as an educational loan.
(1) Qualified education loan The term “qualified education loan” means any indebtedness incurred by the taxpayer solely to pay qualified higher education expenses— (A) which are incurred on behalf of the taxpayer, the taxpayer’s spouse, or any dependent of the taxpayer as of the time the indebtedness was incurred, (B) which are paid or incurred within a reasonable period of time before or after the indebtedness is incurred, and (C) which are attributable to education furnished during a period during which the recipient was an eligible student. Such term includes indebtedness used to refinance indebtedness which qualifies as a qualified education loan. The term “qualified education loan” shall not include any indebtedness owed to a person who is related (within the meaning of section 267(b) or 707(b)(1) ) to the taxpayer or to any person by reason of a loan under any qualified employer plan (as defined in section 72(p)(4)) or under any contract referred to in section 72(p)(5).
