Appearing ofline

Why isn't there a function to appear offline to all or some of your friends?

I have this one friend in League that is always on at the same times as me, he always wants to duo and doesn't really understand if i tell him i just want to solo atm. Makes me feel bad because i feel like he feels bad about me not wanting to play with him.

If i could just appear as offline to him i could play some solo's without feeling guilty.