Is it permissible to waive a debt owed by a friend without informing him, while I still have obligations towards him?

Your relinquishment of your right to your friend is a righteous deed for which you will be rewarded, as it contains a reward for the doer of good and forgiveness for the debtor. Allah named it a charity if the debtor is in straitened circumstances, but if he is well-off, then it is a gift and an act of kindness. And part of the perfection of kindness is informing him of it and having it witnessed during your lifetime.