The main character a bit of a duffer if you ask me. He could've kept his phone recording in the background as part of his rights for every conversation he participates in (with the laws that I'm familiar with at least). This would definitely not absolve him of all punishment, but it shows to a large degree that he was influenced beyond his will. Nevertheless the laws that Dave breaks and admits to in speech to the main character.
The best friend should've done the same thing from the very beginning. I feel that most highschoolers would have this sort of idea (to record conversations that they're a part of- as far as I'm aware is legal) quite easily, especially with the rising levels of 'sexting', (or I guess tangentially following) 'blackmailing', or whatnot. -> This is relevant as this correlates to more awareness.
On another note is that I feel that the best friend should be more grateful than he is for the assistance, but interactions between friends is always an insular thing, so I take it that the best friend is indeed as grateful as I feel is right to be.
Dave is always lording his immense wealth of blackmail material on both the main character and best friend... This could be so easily used against him if easy steps such as this recording idea is utilised.
Another reasonable, but difficult (too difficult maybe), would be to expose Dave's actions to his sister (whom I assume is much more innocent than he is). I feel that he cares for her much at the point of the story I am up to (when main character gets tasked to get a video of Lauren nude), and as such would be a viable way to get Dave out of his comfort zone (whilst also restricting him through his sister- I'd assume).