My response:
Los Angeles is the same way; i.e., the County hires tree trimmers or has their own department (commonly known as "Street Tree" department) because the average homeowner doesn't have the necessary equipment to trim trees.
But, your friend's situation remains the same. You really need to understand the legal term, "Notice". Don't just brush over that term. Without actual or constructive "Notice" on the part of the Governmental entity and / or the homeowner, there just isn't any liability.
For example, a person has constructive notice of a particular fact if he or she has notice of circumstances sufficient to put a prudent person on inquiry as to that fact and, by prosecuting the inquiry, he or she might have learned of the fact. A person has notice of a fact when:
(1). He or she has actual knowledge of it;
(2). He or she has received a notice or notification of it; or
(3). From all the facts and circumstances known to him or her at the time in question, he or she has reason to know that it exists.
A public entity has actual notice of a dangerous condition if it has actual knowledge of the existence of the condition and knew or should have known of its dangerous character. A public entity has constructive notice of a dangerous condition only if the plaintiff establishes that the condition existed for such a period of time and was of such an obvious nature that the public entity, in the exercise of due care, should have discovered the condition and its dangerous character.
However, if there were no "indicators" of a tree in "distress", there would be no "Notice", and there just isn't any liability.
And, to make matters worse, the evidence has been eliminated, and done so for purposes of public safety - - which comes first. Your friend needed pictures and an opinion from an expert arborist prior to the tree clearing to give an opinion whether there was at least "constructive notice" that the tree was rotted, or was in danger of falling due to some other reason concerning the condition of the tree itself.
Without that opinion or proof, your friend has nothing, and will not be able to prevail. Sorry.
On the other hand, there's an old saying: "The squeeky wheel gets the grease." That is, even though the odds are stacked against your friend; e.g., the evidence is all but gone, unless he tries in Small Claims court against the homeowner, he is assured to get nothing.
While I feel that your friend will not, or cannot, prevail due to the matter is an Act of God, he may as well try. It doesn't cost that much to file in Small Claims court against the homeowner and, who knows, a miracle could happen. It's doubtful at best, but at the very least, your friend will get a small lesson in legal reasoning and knowledge of the applicable law from the judge.
He may as well give it a go. He's got nothing else to lose by doing so.
IAAL