It is no longer a legal deer once you tresspass onto property you do not have permission to be on.
My take. In this particular situation, the hunter did everything right about contacting the park ranger to assist with retrieval of the deer. In that situation, the hunter can show the ranger the exact tree he was in, and the blood trail starting on the private property leading to the state park. Once verified by the ranger, he should have used his judgment and recognized this was an honest hunter who was playing by the rules, and then should have helped him drag his deer off state lands and back onto private property.
Totally different situation if he had been caught illegally hunting or illegally retrieving game off a state park.
Been in the same situation myself as the landowner. Caught poachers hunting on my property, shooting game on my property, and people retrieving game on my property without permission.
All the above got prosecuted except for one last year.... neighbors sons friend who gut shot a deer, then drove his truck through my winter haylage planting and rutted the field to begin his search, and I caught him at 10 pm destroying the blood trail through my woods. Prosecution would have caused too much bad blood between neighbors, but I asked for restitution for the destruction of the parts of the winter crop he ruined.
The flip side... had a few call me asking permission to retrieve game that ran onto my property after being shot. If you call first, I'm ALWAYS going to allow you to retrieve your game, and if I'm close by, I'll even help you drag it out or to one of my roads if it makes retrieval easier. IF you ask for permission first.