So you haven't seen the actual credit report from either husband and wife and just know what the tenant's realtor told you? Are you sure there isn't anything else that is outstanding?
I highly suggest that you pull your own credit report on both tenants yourself and examine what is on it and not go by the report of a third party who is looking out for the interests of her clients which is not you but them.
You have to look at the big picture. What kind of other debt do they have outstanding besides a student loan, which is not necessary a huge red flag if they have the income to cover the payments and no other high balance debts?