Its really a case of the riders skill level and money they can outlay, if he has the money for the DAS I would suggest he goes right for it because he will get a full unrestricted licence and then can pick up a 250cc bit more poke than a 125 but not going to set the world on fire. Get a cheap Honda CB or similar for a few months and move upto a 600cc for a year then do what you want.
If he just wants a taste of biking and does not want to lay the money on the line before he is sure, you have to do the CBT anyway as part of a DAS, so he can always do the CBT and ride as a learner for a few months on a bigger 125 like the Varadero XL125 (Which are awesome bikes). Thing is different riders have different skills and habits, if you have been driving a car a long time you let bad habits get ingrained into you some of which don't translate to motorcycle riding.