You know that the author of the letter is a fraudster and, hence, a suspect. However, it is unlikely that the name or any other information provided is valid. Hence, the author is not a suspect you can identify. Considering that person does not rise to the level of an identifiable suspect, your threshold for filing is $25,000.
Next, you consider your customer. She is clearly known to you and the issue is whether you think she was ignorant or dishonest; i.e. was she the victim or the accomplice? If you believe your customer was the intended victim and her participation was unwitting, the threshold for filing remains $25,000. If you believe your customer was dishonest; i.e. a knowing accomplice, then your threshold is $5,000.
The key to any decision to file or not in these circumstances is your conclusion on whether the customer was the accomplice or the victim. As the fraudster is usually unknown, it's your decision regarding the customer that dictates the filing threshold.
The resources rlcarey linked above clearly indicate that even in circumstances where the amount involved does exceed $25,000, sometimes the relevant agencies simply don't want to hear about it anymore if there is no loss.
_________________________
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.