Speaking as someone who went straight from training into a co-pilot job on a medium twin, i then found it comparatively easy to go back and get the single pilot "Jet-ranger" type experience that then gave me enough P1 time to get a command in the medium twin. It's worked out very well for my career.
From an airmanship and learning point of view, yes, it would probably be preferable to get 1,000 hours in the 44 then switch to the 412, but we don't live in a perfect world and if he has the option of getting a 412 rating and some p2 time then i would take it and accept that there are different ways of gaining experience.
The 412 rating is very likely not to be an option in the future - take it now.
Best of luck
Barny