If a customer has forgotten their ticket there are a number of approaches you can take depending on your policy. If your policy is that the guest must show their ticket on arrival, then you can refuse guests entry to your event on the basis that they must show a valid ticket.
If you have the OutSavvy app or guest list printed you could decide to check the guests id and check them in manually on the app or cross them off the guest list.
Otherwise, if you do not have the app or the guest list it is at your discretion to allow the guest entry to the event.