It's a Bitcoin mixer. They have coinjoin and advanced feature of mixing bitcoin. The audit job is to try to track back the mixed funds.
You all must be familiar of coinjoin so nothing new to explain.
Their advanced feature is to mix your bitcoin with XMR and then from XMR to bitcoin. You don't need to do it by yourself, when you chose the advanced option then the system does it for you. At least this is what I understood.
While code auditor play role on that job, i think what your client actually need is expert on digital forensic/forensic analysis which have experience on cryptocurrency field. And since many mixer/coinjoin technique rely on amount of user to improve it's privacy, audit result could be less useful when your client have lots of costumer.