I had the client scan and email the letter and it appears to be authentic. It is from the IRS is Austin, TX and references the Integrity & Verification Operations Division. You are encouraged to chose the online option. The IRS provides additional information as to what information you should have when you logon. It seems to me that they are trying to look for identity theft. 

The alarming part is that this is going to hold up the clients refund for another 9 weeks.