I towed my 26Sonic from Maryland to Florida with an 02' that had the 5.9. I think it struggled quite a bit, had to take off overdrive for the entire trip and it ate gas like crazy. I normally tow with a PowerStroke and it's not even close.
I would tell him to go with a Duramax, PowerStroke or wait for the Ram 2500 with the Cummins to come out, if it hasnt already. Yes the diesel's are a $4000 option, but believe me, you will save it in the long run and you wouldnt even be able to feel that boat is back there.