if we exclude bitcoin from the list then it depends on timing. there are years (like this year) that altcoins are dumping hard and everyone loses money even miners so they will have a much harder time reaching profit let alone ROI. of course with no electricity cost it will be better but not by a lot.
then there are years when we have lots of altcoin pumps like 2017 or 2013,... and mining coins, specially the GPU coins (since they are pumped a lot more) is more profitable than coins that are big enough to have an ASIC.
I don't believe on altcoins honestly. We see altcoins on the past massively increases but I don't know if it can happen again after the block halving if miners can hold their precious altcoin why not. Since he has a free power source they can mine 24/7 without thinking about electricity cost but we don't know if that coin will massively increase just like what happened in the past there are some coins die before it happens. So instead of holding them, they sell their coins before it dies.
Mining altcoins is just gambling if you invest right now with free electricity you can gain profit but it takes years before you reach your ROI like what happened to me I invested my money to buy a small rig with the small solar system year 2018 until now I can't reach the ROI. I'm just holding and sell if the price increase 100% or more but still I'm far to reach the ROI because I only have a small solar system with two 9kw/h battery bank my miner runs around 12hrs to 19hrs. Even I have a free source I'm too far to get my ROI. I'm waiting for block halving and holding but instead of waiting for too long I trade some of my coins to major altcoins so that I can make a little profit while waiting but not focusing only on one altcoin.
That is why I suggested him to buy ASIC because you can get your ROI fast compared on mining altcoin. Why not he buy ASIC first then after block halving start mining altcoin then hold so that he doesn't waste the opportunity to make a profit these months. Block halving still too far so this is the best choice that I know if he is looking for a profit and become rich after block-halving.