Getting Started as an Ad carry...

So, I have introduced my friend to league not long ago. He is now level 28 and wants to main Adc. He has learnt the basics such as csing, what to buy and basic mechanics. However what lets him down is ability to keep constant farm through out the game. I'm not entirely sure how to make him improve on this. I would appreciate if anyone has any advice or tips for this. Also, since I don't main ad carry I can't really show him everything as I main Top myself. So any additional tips, or maybe some adc challenges would be awesome if you knew any. Thanks.
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