I think he should have done something more constructive with the laptop, such as give it to charity (that would have stung the girl just the same), but either way, I can totally understand where he's coming from. If I had a kid who went on Facebook to belittle me after all I've done for them... yeah, it's on.
Let's not forget that a lot of kids are like this:
Okay, firstly if he wants to shoot a laptop he bought her (and then spent several hundred dollars upgrading and working on in the process of finding this letter), he has the right. I agree with Rob though, it would've been better off if he just donated it to another kid that works hard and doesn't have access to one.
After he read the letter I can agree with his position, being grounded for 3 months for a previous, similar incident and apparently having ignored the grounding by posting this letter and using the net anyway... she was going to lose the laptop regardless no matter what. She clearly can't be trusted with one.
Given the language and her diatribe though... it's clear she probably still isn't going to learn a thing from this. Except maybe how good she used to have it.