I might have miss read it but I get the impression that you don't quiet get the whole picture. Maybe you do but I might be way off.
What I mean is.
*chop*
You must do all of these things in a twin, which means there will be these expenses of doing x-c in a twin.
Just giving you heads up in case there was a miss understanding.
So even though you don't need to do time building, you will still have to meet these requirements and these expenses.
Some of those things can be done at the same time. Things like his 300nm "simulated solo" xc can be done at night. Then he can spend 5 hours in the traffic pattern doing landings and takeoffs at a towered airport and knock out those requirements. (10 hours of "simulated solo" time)
Some of the instrument training can be done in a cheap 172 (only 5 hours needs to be done in a twin).
The complex takes care of itself since it's a Duchess.
The 50 hours of PIC XC do
not have to be in a multi-engine airplane. Just an
airplane. He meets that requirement.
He'll have to do the dual VFR cross countries in the Duchess, but he'd have to do that in a 172 also..it's just a little more pricey doing it in the Duchess.
It's about $800 more expensive for him to do the multi first then add the single. Unless he can do the single add-on in fewer than 5 hours (possible), then it's even less more expensive (if that makes sense).
If he has a good instructor (and a "senior" instructor as they're called sometimes) and does his commercial multi and single in the right seat, I'd bet he could knock out the CFII & MEI in the same checkride in the duchess (after getting his 15 PIC Multi...he could work on both the CFI-IA and CFI-AME during the 15 hours of PIC time) then doing the CFI in a straight legged 172 with no problems. Figure 18-20 hours for the MEI and CFII in the Duchess then figure 5 hours for the CFI in the 172. That should take care of it all.
20*205 (Multi including checkride) 4100
15*50 (CFI) 750
5*90 (172) 450
3*50 (CFI) 150
~$6,000 for all three CFI ratings (plus examiner fees). Not bad.
-mini