Bah, AC/DC became a cover band already when they replaced Mark Evans with Cliff Williams!
But seriously, if AC/DC was a cover band now, how come that the Young brothers went from being in the band they "always" have been in, and then suddenly become a cover band of themselves?
A band doesn't become a cover band of themselves just by some change in the line-up.
Otherwise, it would just be cover bands out there. At least those worth mentioning.
As for your friend whose logic follows that Brian Johnson can't sing because he thinks that all AC/DC songs sounds alike, well... He's entitled to his own silly opinion, just like any other tone deaf heathen with knitting pins protruding from their bleeding waxfilled ears.
But what does "Who Made Who" got to do with who's the singer of AC/DC?