It's all about VPN. Always ask the booster if they use VPN or not. If they don't, AVOID absolutely.
We provide boosting services from October 2014 (although we're pretty new to this site) and we never had one banned account, but i know about people who have been banned because they used to get boosted from people who didn't use a VPN service.
This is not gonna happening if you get boosted only once, but you have to wait and not log into your account for a few hours (better one day) before and after the boost, to make the IP change possible and not an evident proof of boost.
EDIT: Riot do not check player's IP but they automatically check huge changes of IPs (example, Germany to USA, or France to China, France to Poland..)