Interesting point and maybe I'm forgetting some part of the story but for the life of me, I cant remember who has David shit all over?
However I'd read a couple of stories where the central character is meant to be a wonderful person, yet carries on like spoilt child.
And yet it appears from your comments that's what is expected. Being strong appears to require a slash and burn response to any transgression- real or even just perceived. Not responding that way is considered weak.
I get him staying friends with her to a small degree but allowing her so much access to his life and business ventures is just nuts.
Each time she's screwed up, he's distanced himself further from her.
Personally, I would have cut all ties after the first time. But David isn't me or you- he's him.
They problem I have with David as a character is hes ment to be a teenager but is written like a very wise almost Yoda level adult for the most part.
That's the thing with stories such as these.
Do you want a story about your normal average kid? One that acts just like a normal average kid?
There have a been a few stories where the kid behaves pretty much like normal kids, and people respond even more frustrated than you are with David. "How can they be so stupid? Why do they keep doing dumb things? Why did they do that? Anyone would know that would lead to trouble!" That's just it, normal kids do normal kid things which tend to be stupid. If they're lucky the results aren't too painful or fatal.
Remember the beginning of the very first story? David was a normal kid, trying out booze & drugs. And someone nearly died, and he got deep in the crap & went to his Uncle's farm to get sorted out. And then buried in the avalanche and had given up hope of rescue.
Things like that change people. He could have become a PTSD nutcase/arsehole, luckily he became stronger for it. David has been through & survived & recovered from some crap that most people, let alone a teenager, never have to deal with.
So as a result he isn't the type to just write someone off because they make a mistake or 2- he once wasn't any better than them, and he knows it.
I just find it odd that people consider such a response as weak. Dealing with the stupidity & neuroses of others in an adult manner takes a lot of strength & patience.