I would put $1 a day in Cryptocurrencies, but I would not actually invest daily but rather make a weekly, monthly or quarterly purchase instead. That is due to probably insane transactional taxes that would leave you with probably less than 75% of the actual investment.
Besides the trading fees and the withdrawal fees if he wants to withdraw his coins to a private wallet (which is always recommended), there are other limits to consider:
- The minimum trade amount on the exchange you are buying on. Chances are, you can't buy $1, $5, or maybe even $10 worth of BTC. Maybe their minimum is $20 or more.
- The minimum withdrawal amount on the exchange. No CEX (I am using CEXs as a reference point since they are mostly used for buying/selling crypto) that I know of will allow you to withdraw $1 to $5 in BTC. Maybe not even $10.
For higher ROI don't take the bet on BTC only, yes it's the safest option bit for better gains you need altcoins as well, even Ethereum would do just fine, now that Pos and the better gas fee are around the corner Ethereum will perform very well from now on.
You can't possibly know what the future holds for Ethereum. No one does. All we can do is speculate. The value of ETH could fall once they migrate away from PoW. There will no longer be a need to invest in expensive mining equipment. Miners become validators and stakers who will profit even when they sell their ETH cheaper. That can drive the price down.