How do I convince my bronze V friend to take Flash ?

So I have a "friend" in bronze V. He has started to not take flash on nearly every champion he plays in ranked, inless i'm in the game. On veigar, he takes barrier ignite. On ahri, barrier ignite. On ryze, barrier ignite. He can't seem to agree that flash is good, you can flash over walls to save yourself, to chance, flash 1 hit enemy adc then zhonyas, outplay, dodge core skillshots, etc. He is an -extrremeley- stubborn person who belives even faker can't carry his elo ( Yes, he believes if faker was in his games he would still not win. He is stubborn on everything, summoners, builds, champions, etc. It took me weeks to convince him to start buying QSS as an ADC, especially since it was buffed. How does one deal with someone so stubborn?
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