I guess it depends, but yeah if he contacts riot with proofs its possible he could get you banned. I wouldn't say 100% though.
So basically i was being greedy and i went for completely new booster, and he did boost me, i paid him all was ok, but he didint finish his boost, and i wrote kind of negative review on another forum about him, and in case he reports me , as he got all chats where im giving all my details to him, what's the ban chance for me ? pretty much 100% ?
I guess it depends, but yeah if he contacts riot with proofs its possible he could get you banned. I wouldn't say 100% though.
If there is solid proof you will be banned, yes.
@erniuss If a booster anomonyously reports you for boosting your account your account might get banned or not depends if he actually reports you. Riot Games dont take boosting kindly so ... chances are pretty high if he does report you.