I've ran into this exact situation. I told and showed my customer exactly what happened. I explained the only options: buck up and buy the door OR I offered to super glue the piece (customer found it in their cup and kept it) back on. I was very clear that I couldn't guarantee the super glue repair at all...and I documented that on the invoice that they signed. They opted for the super glue. It was a pain in the arse getting everything to line up while holding the piece while the glue dried. I used Gorilla Super Glue. It worked great. Told the customer to be very gentle when they press their cup in there. I never heard back from them...no news is good news I suppose.
If I remember right, that whole assembly was available, but very expensive. Double check on the current availability of that assembly rather than the whole door.
Good luck! Let us know how it turns out for you!