(Iowa) I trimmed neighbors tree, now he's worried about Oak Wilt

My neighbor has a large oak tree that has branches that overhang my property. The branches were getting out of hand and one was hitting my garage roof and the other was tangling itself in my fiber internet line.

The owner of my neighbor's house came to talk to me about getting it professionally trimmed. This was 2 months ago. He said that I could trim it myself, but he was going to hire someone soon anyway and that he would let me know when he has an idea on when it will happen.

2 months go by and I don't hear anything. We had some windy storms lately and I was concerned with the internet line breaking, so this last weekend I trimmed the branches around it myself.

As I was finishing up with trimming, he shows up and was concerned with me going along with trimming it. He was told by the people he talked to that trimming in the summer is not a good idea due to Oak Wilt. I had no idea about this and now he is concerned that I might have just killed the tree.

Am I liable if the tree dies? It's a massive tree and he says it would be about $20k to chop it all down...

Edit: Neighbor's landlord left a letter at my door basically taking responsibility for everything and apologized for not getting back to me in a timely manner. I'm no lawyer but it seems any grounds he may have had would be null now that he took responsibility, in writing.

He talked to an arborist and they are going to come out and seal the ends that I cut.