Neighbor advice

Katethegreats post about her trespassing neighbor got me thinking about something.  About a year ago MH came home from work and our neighbor was outside.  She waved and smiled and said I trimmed your tree for you!  To MH's horror he saw that it was butchered.  The identical tree 10 feet over was full and leafy and this was one hacked to death.  The tree is on our property 100%, nothing hangs over onto her side, she had absolutely no reason to touch it.

We are also not friends with these people, so I can't fathom why she thought it would be ok to do this.  At best we wave and smile when we see each other.  They are foreign and there is a bit of a language barrier, so I have no idea if where she is from this is ok to do, to go onto your neighbors property and hack their trees.

 MH was seeing red, but his bark is worse than his bite and pretty non confrontational and never ended up saying anything to her about it.  By the time I realized he wasn't going to do anything, it seemed kind of too far past the fact for me to say anything.  My dilemma is the landscape we plan to do this spring.  We want to plant a tree in memory of my grandparents, so this tree will be A. expensive, and B. sentimental.

 I think I would lose my mind if she touches this tree due to the cost and the sentimental fact, but is it weird to say something to her almost a year and a half later?  MH thinks we should say nothing and if she were to do something to it, expect her to pay damages and take her to court if necessary.   I don't want to risk it coming to that, but I also agree you shouldn't have to go around warning neighbors not to touch your stuff.  And like I said, I feel awkward saying something after so long, and I am a fairly non confrontational person unless I am pissed off enough that I just react before I can think about it.  What would you do?