It is always a good idea to document the fact that the customer withdrew, but, as David points out, you don't have to send the withdrawn customer anything. This is one of the areas where creditors indulge in over-compliance.
There is also the provision in Reg B's adverse action requirements that provides for multiple creditors. 202.9(g). Under that section, you wouldn't have to send the customer a notice even if you had turned them down because they got credit from another creditor in the arrangement.