I think the guys utilisation profile would need to be expanded on to give a proper recommendation. Is he looking at an aircraft to cover the Navajo whilst on Maintx?
If this is the case, it is unlikely that this amount of utilisation will be cost effective to purchase another aircraft, it would most likely be better to cut a deal with a local air taxi / charter operator for the couple of days a month utilisation.
If he's looking for a new "big boys toy" why not trade in the Navajo for a BE-90, BE-200, Meridian etc etc.
In reality the Navajo / Chieftain / Seneca and othe r piton twins are great value for money, but in my experience are only cost effective to operate if you have multiple aircraft of type and maintain them inhouse.
As Egeebee says the Navajo and Seneca's are old hat (but very cost effective for short trips if your budget is low) The BIG question is, how much does he want to spend, both on capital purchase / and or monthly, hourly based on What utiilis