He informs Revenue of the new employment ( give him your employer's reg'd number ) . He informs them of how he wishes his credits to be split. You, the employer have no role in this and cannot arbitrarily decide to split his credits. Until such time as you receive a certificate from Revenue for him, you must adopt the normal emergency tax procedures ( per the Revenue's [broken link removed]page 42 part 7.8)
Remember the employee may wish to retain all credits against their other employment ( depending on their tax position they may still earn enough in 3 days to absorb their credits ) and in that instance they would be taxed in full on your employment.
It is dangerous to make any assumptions without proper documentation. Adopt emergency procedures and if there is any tax adjustment on issue of the Revenue cert it can be made then.