Is it fraud if a person is cashing checks for another person because that person owes the IRS money?

Conspiracy to commit tax evasion and conspiracy to defraud the government. Both are felonies and both carry the same penalty as the principal actor committing tax evasion and defrauding the government.

In the US if you knew the person was having you cash checks to avoid taxes, you will be charged as an accomplice

If you unknowingly did this then you might not face charges but the fact you are posting here means you know what you are doing and this post can be used as evidence in any trial

A partner to the crime in this case yes just as guilty as the first person is when doing this for that purpose.

Hope that you find the above enclosed information useful. 06/24/2014

like a lot of the Nigerian scams, at some time one or more of those checks may not be good and that will mean you are out the amount the bank takes out of your account and the amount you gave he when you 'cashed' her check

and yes, if the intention is fraudulent you are complicit in the conspiracy

Its called complicity to fraud. They're not committing the fraud. But they are assisting in the furtherance of it. Just as culpable criminally.