There are books on how to build altcoins but I don't see any in-depth guidance on how to build a gambling site.
It seems to be all trial and error.
If you know of any technique or site or forum I would appreciate hearing about it.
Do you intend to do that yourself?
I am not sure he would be able to build a whole gambling site by himself if he doesnt first have the technical knowledge, gambling application is not so easy like building an ordinary website, it requires lots of coding to do so, and I am not also sure that only him can do it alone, the best would be for him to hire application developers that would do so for him, and if he intends to run it on blockchain technology, he might have to hire a blockchain developer.
If he has a bit of the knowledge, the best place for him to learn more is to follow tutorials for doing so on YouTube, I am very sure that we have lots of gurus that would have upload a video on how to do so on YouTube and give him other great ideas.
Even they have tutorials in youthbe that is not enough to create your own gambling sites. The needed for that is a person have profession in proragmming or having skills in computer like coding , designing etc for he able to build his wants. I think better if you do create at your own so you can save more money than if you hire someone but if Op still not know maybe the alternative way for that is to hire professionals.