Is it possible to gift a skin to someone right after the game?

Or you have to be friend with him for some time? Last night I prommised my team mate Nasus a skin because he vas carrying us so hard and I was in my series. He then told me its impossible because we have to be in eachothers friend list for about 30 days. Why is that a thing? It would be so cool to reward someone with a skin if they deserved it.
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