According to the IRS, if you have a CAF number on file and in good standing, there is no need to file the Form 2848 or Form 8453. Rather, they say to cross out the tax preparer info designation and write "POA" in its place with the CAF number. Then sign it on behalf of the filer.
However, Turbotax doesn't seem to have a way to indicate you're filing for someone else or the best way to general the signature docs. I've searched for a consistent answer for this and haven't found it.
