Personnaly, I think I could see my 4 1/2 yr old doing this now & even in a few years... they don't always think before they react.
We hid our 3 doz eggs inside this year - some were really easy others a little harder... I have a 2 1/2, 4 1/2 & 6 yr old. My 6 yr old did push her baby brother out of the way to get some of the easy ones. After the older 2 gave up - I helped the littlest find the harder ones. They all needed found - they were hard boiled eggs.
But if there is no real boundry marker - it is harder for little kids to know what is their yard & what isn't. And when they are excited - their brain goes out the window. I'm not saying what he did was right, but I do feel his punishment was a good one. Even if your daughter was a bit older - she could have been told that he found some in your yard by mistake & will rehide them for her.
My kids took the plastic eggs out of their baskets & are still hidding them all over the house... they are playing Easter Bunny - even if they are all empty. They don't care - it's just fun to hunt them... so they take turns hidding them.
One question - how well does this boy know your family? I know one of my neighbors very well, but on the other side - I don't know if they have a kid or not. They never leave their house - unless they are leaving & even then they don't say anything if they see us. Heck - we just found out that one of the kids in my daughters kindergarden class lives 3 doors down last week & they have lived there for 3 years.
Either way - as long as he knows he did wrong & accepted his punishment... all has been handled. And everyone should leave it behind them & laugh about it down the road.
Glad you are starting to see the humor in it & I hope you neighbor boy's mom does here soon also.