First, he has to have enough “equity” in his home to take out the extra money. Second, he should be sure that he is getting a better deal on the conventional loan — and that he won’t have to pay Private Mortgage Insurance (which could raise his monthly payment).
But THIRD — and very important — as of this year, you can no longer deduct the interest on that portion of the loan where the proceeds are used to pay down student loans or other debt. You can only deduct the interest used to purchase or substantially “improve” the home. So a portion of that new mortgage payment (which is mostly interest in the first years, anyway) would no longer be deductible.
He needs to do the math very carefully on this deal, because there are a lot of little ways it could become more costly. Ask a CPA for advice on this. He may be better off trying to refi his student loans to a lower rate at a place like www.SoFI.com