Hello there! If you archived an Instagram photo that was co-published with one or more individuals and then removed the collaborator from your following list, the photo may no longer be easily accessible. Archiving posts essentially hides them from view but should still be retrievable through archived posts section; if it cannot be located there then chances are it has been permanently deleted from existence.
Now that you are friends on Instagram with your collaborator, it may be worth reaching out and explaining the situation to them. They might still have access to a copy of the photo or can give more insight as to its destruction - plus they might just have removed it themselves; so having an open dialogue could shed some light into this scenario.
Hope everything can be resolved successfully and that the photo you worked together on can be recovered!