There is no textbook to tell you what to do when a client has agreed to follow a certain process, but does´nt follow it.
There are many reasons why:
Production: Some clients believe that cutting corners or taking less time is productive, and it might be at the time, but that is only short term, it eventually will come and bite them.
Cultural: There is a cultural ignorance in following process, these can last for years, and cost many thousands or even millions, this cost somehow has been a custom and therefore part of the culture of the company
Its “LEAN”: Some of them confuse “Lean” with an actual value process. This has to do with the top two reasons, they believe it’s a “waste” because they are not accustomed to it and believe its non productive
What to do?, its actually mind boggling how these processes actually work in favor of the client but same client refuses to follow them, this has been what we have done when this type of things happen
Cost Risk: Inform the client of the risk he is taking, there is no way of convincing him if this actually has ties to a cost, so make that risk tangible and it could be a time risk or quality risk or scope risk but at the end of the day everything should and can be translated to COST
Do the work yourself: Sometimes you need to prove to the client that the process works. Do it for a couple of times highlighting what risks you prevented from happening
Have a 1 on 1 with your client, if a process is not followed, then nothing garantees that others will be doomed as well, and this could be a pattern for disaster, let the client know that this is important for your work
Lastly, dont shy away from confrontation, sometimes the best relationships come after them, and sometimes the worse relationships are without them. Surely with confrontation you run the risk of getting fired, but that is the risk you will have to take always to be a better manager day in and day out.