Well it's HIS playing, if he's okay with it... why bother? If he doesn't ask for advice... (Unless he's in your band...)
He'll eventually agree on his shortcomings if his stuff reaches a wider audience or he's working with professional engineers/producers who'll be telling him that his performance is lacklustre.
So, jut give it some time. Even Steve Vai COMPLETELY changed his picking technique after decades of playing with the biggest cats in rock and putting out solo records...
He claimed that I was the first person ever to comment on his playing in a negative way. However he seems to respect my opinion and he asked my advice and he does try to improve his playing to convince me he is as good as he thinks. Still, he doesn't fully admit/understand the shortcomings of his technique so there's not any significant improvement.