Exempt Salaried Manager who is no longer with the company had a lawyer send a letter stating that unless she was paid for hours worked in excess of 40 hours per work week that he would turn the case over to the labor board. In the 9 months she was a manager she worked approximately 100 hours of overtime. She was terminated for falsifying her pay records (which she admitted to doing over 20 times and signed a document stating so) so it is difficult for us to access if the overtime is accurate. Do we need to pay her this overtime? We have never paid any overtime to anyone who is an exempt salaried manager? Should we be concerned?

As long as you feel you can legally justify her exempt status, I wouldn't be overly concerned, no.

Which raises the question, can the OP legally justify the Exempt status?

Exempt Salaried Manager who is no longer with the company had a lawyer send a letter stating that unless she was paid for hours worked in excess of 40 hours per work week that he would turn the case over to the labor board. In the 9 months she was a manager she worked approximately 100 hours of overtime. She was terminated for falsifying her pay records (which she admitted to doing over 20 times and signed a document stating so) so it is difficult for us to access if the overtime is accurate. Do we need to pay her this overtime? We have never paid any overtime to anyone who is an exempt salaried manager? Should we be concerned?

Look up the rules and make 100% certain that the 'Exempt Salaried Manager' really falls into one of the Exempt categories. The rules are very complex and arcane and it's easy to miscategorize someone. Just calling someone an Exempt Salaried Manager won't cut it. Not all managers are Exempt, either.