Hm I don't think you can just allocate 30% for expenses without knowing more of OPs background. It is unclear whether the $600 are already for investing or whether it is just the salary.
So first, OP please clarify if you want to invest $600 or whether this number is smaller as you have to deduct your expenses first.
Apart from that above reply already gives you an idea what should be done: split your investments in different asset classes and only allocate a small amount for crypto were a 100% loss would not bother you
It may just be the salary.
We don't know if the friend of OP is paying for a rent, own a house/with parents, with insurance, healthcare, and other stuff that is being paid monthly like bills, electric, water and internet.
But if he can save like just 10% percent of that, $60 to be precise, that will be a good number in the long run. No one is rushing when it comes to saving money thru cryptocurrencies especially in Bitcoin or Ethereum. It may be the best window now for the cheap price and you can maximize the amount that you can grab.