Find the amount that changed for everyone
Imagine a shop offers free delivery for $40 in food. The three diners originally selected $15, $14 and $13, for $42 total. If the $13 order drops out, the remaining $29 may now carry a $6 fee. Recheck the actual basket before assigning the remaining diners their totals.
Ask the remaining people before checkout
If they still agree to divide delivery equally, the $15 diner owes $18 and the $14 diner owes $17. The new total is $35. The person who withdrew should not be assigned a share automatically when no order was placed and the group had made no commitment.
Treat an order already placed differently
Check the final total after the change. The order might no longer meet a discount or delivery threshold. If that happens, share the new amount before confirming. A small menu change can alter what everyone pays, even when their own item stays the same.
If the restaurant has already accepted the order, first find out whether a change or refund is possible. Then apply the group's original agreement to any cost that remains. Keep a record of the final order confirmation; a disappearing item in a draft cart does not prove that the restaurant canceled it.