What he doesn't realize is that they usually have a image of the SSD and every night the SSD is completely cleared and set back to stock.
They do this because people download viruses and other malware and it just saves them the hassle of having to do it manually.
So even if he did install Nicehash software on a few PCs, the mining would stop usually at the end of the day.
That or since most GPUs use tons of power, they would end up blowing the breakers at the end of the day and the owner would get suspicous.
This is probably true for those higher end internet gaming cafes. However, in some places i know, either there are simply too many customers waiting to use computers to waste time doing backups or the owners / shop keepers barely bother to do it daily (they do it as necessary).
That being said, i believe the real question here now would be profitability. Gaming PCs are not optimized for mining and each would probably be using components that are not power efficient, and could possible even losing money on electricity costs. Note that each GPU here is also paying for the cost of CPU, RAM, etc which is inefficient compared to 13-GPU rigs that share the burden of electricity cost.
Optimization does not matter. For places that have 1070tis, etc, and have a lot of downtime (which they do here), they will make good money long term. We are just opening nicehash and clicking go. Power is also free (in the UAE, government subsidized).