Don’t feel bad about not understanding grounding and bonding of panels. I’ve been teaching apprentices, journeymen, and electrical contractors for many years. You might be surprised to know that even many experienced master electricians and electrical contractors do not understand the rules for and bonding sub-panels. There are some instances where a sub panel can be grounded the same as it would be if it were the main panel on a separately derived system.
The NEC is not a design guide. It is a set of rules. I would not recommend trying to learn from the NEC. There are many good books available but my all time favorite is “Practical Electrical Wiring” by H.P. Richter. I’m not much involved with apprentices anymore so I don’t know if it is still being published but, that was always the first book I’d have them read.
The important thing to know is that even though most subpanels are required to have the grounded and grounding conductors isolated from one another, that is not always the case. So, be careful how you report it to your client. Don’t list it as being a deficiency unless you are sure. If you are not sure, recommend evaluation by a qualified electrician.