It can be done, but carries high risk. The IRS generally copies the taxpayer on correspondence with the POA, meaning that the taxpayer is likely to find out that something has occurred without his/her permission. Once the taxpayer contacts the IRS and the IRS finds out the fraud/forgery occurred, the person who mislead the IRS is likely to face prosecution. So as I said before, it would be foolish to do it.
Of course everyday we find people who do foolish and stupid things. Unfortunately, as advanced as we are, we have not yet found a way to eliminate the knucklehead gene from human DNA.