- CODE OF CONDUCT
5.1. Can I troll, flame, threaten or harass people while using the Riot Services? (No. If you do, you might get banned.)
While using the Riot Services, you must comply with all laws, rules and regulations in the jurisdiction in which you reside. You must also comply with certain additional rules that govern your use of the Riot Services (the “Code of Conduct”). The Code of Conduct is not meant to be exhaustive, and we reserve the right to modify it at any time, as well as take appropriate disciplinary measures including account termination and deletion to protect the integrity and spirit of the Riot Services, regardless of whether a specific behavior is listed in the policy as inappropriate. In addition to the Code of Conduct, please review the Summoner’s Code for additional guidance on exemplary gameplay behavior.
xiii. Playing on another person’s account to “boost” that account’s status or rank.
Found this on the T&C's. Even if you were to use the account, because it was not originally yours then you'd get banned on it anyways.
http://na.leagueoflegends.com/en/legal/termsofuse#code-conduct