Bert was a personal friend of Carter, with good practical wisdom to balance Jimmy's more cerebral approach.
As head of a major bank in Georgia, Bert was criticized in '77 for using the bank's jet to attend football games. He was also criticized for arranging loans for friends of bank officers and employees. This one's potentially more serious. Maybe it hinges on how far outside the norm the friends are as far as having conventional credit ratings. If it's close then you can argue that your personal knowledge of the person's good character, etc, etc.
So, what if Carter basically pulls a Bill Belichick?
He says, I'm not going to comment on an ongoing investigation. And he keeps Bert Lance as long as he reasonably can.