EntityItem does have thrower and owner fields, but thrower is only set when a player drops the item using the Drop Item key (Q by default) and owner is only set by CommandGive (the /give command). Minecraft doesn't track the entity that dropped the item in any other way.
If you want to track this yourself, you could store the owner's entity ID (synced between client and server, not persisted) or unique ID (not synced between client and server, but is persisted) in a Capability of the EntityItem. You can only reliably gather and store this information in LivingDropsEvent, which isn't fired for non-living entities.
Please don't PM me to ask for help or to join your mod development team. Asking your question in a public thread preserves it for people who are having the same problem in the future. I'm not interested in developing mods with people.