As someone who has seen many Green Herons in the wild over the years, let me make some comments:
1) I have never seen one in the wild being anywhere near as tame as the one photographed, they invariably fly around here with even a single person within 10-30 meters, unless the cover is so thick they feel unobserved. I have seen photos from more urban areas of Green Herons allowing people to approach closer, I assume because of habituation. That bird must be very, very used to having people close by, because in my opinion, that tameness is not normal in a wild Green Heron.
2) I looked at a number of photos in different lighting conditions, and I don't like the tone of the brown on the neck. It is too grayish, too flat and cool in the coloration, the whole bird looks flat and too gray in fact. A green heron should have brown in the throat stripes that has more of a warmish, chestnut hue to it -- and the rufous on the solid patches on the sides of the neck should generally be more extensive.
If I was you, I would be taking a long look at some of the 20 different subspecies of Striated Heron, that were split off from Green Heron some time ago.
Also add me to the list that finds it unlikely to the point of being impossible for a wild American Green Heron to have fresh juvenile plumage in Europe in April...