For the most part this is sound advice, but there are still opportunities, even when alts are over inflated (if you can catch the right ones at the right time). BTC is on the rise again and will probably dump once it hits $20k, $25k, $30k etc. leaving lots of opportunities to grab alts. Your friend will probably lose money short term, but a few months from now, he'll still probably make a reasonable profit. I sat on my Ripple for 4 months while it stagnated, thinking I'd made a mistake, but it proved out in the end.
$25k and $30k are not possible targets for the near future. They're for Q2 in this year. When this happens, the price of altcoins will be dumped easily. This is a fact. And it's better to have some bitcoin in your portfolio, even small amount of it.