The most important thing is not to ignore the fact that your client has ignored your advice. A client ignoring your advice could have a material impact on the investment advice you gave.
For example, if you have prepared a financial plan with certain assumptions such as cash flow, a customer who alters the cash flow could impact the overall plan. So what do you do?
The critical thing to do is to document the fact that the customer has ignored your advice and detail the ramifications for doing so. Using the example above, you should write a letter or email to the client explaining the advice you gave (including whatever assumptions it was built upon) and detail the impact to the overall financial plan.
This is no guarantee that the client will not complain at some later date, but you have a paper trail. You documented the advice that you gave the disregard of that advice, and the impact for doing so. The only other question is whether to fire the client.
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