Don't give your username or password to your friend or anyone else.
Now you know the terrible truth: FREE RP is never truly free.
To be completely blunt, Riot Games will never have an event where we will require your information to give you rp, as we can just credit them to you automatically. We don't need a third party to do it and we definitely wouldn't need to share your info with anyone, ever.
My trusted friend that I know in real life told me that this free rp offer is legit!
Would you trust your friend to play Yasuo in your promos?
A completely trusted source is offering me RP in exchange for my information!
Why can't they gift it to you instead?
I am convinced that I got a free RP offer from Riot Games!
Chances are "we" asked you for your username and password (Pro-tip: That means it isn't actually us).