SoCall App, fair go. I guess there was wrong presumption on both sides.
I know what you mean now and agree. J1 internship is normally used during of after Uni degree or training of certain length/level and/or work experience, to broaden one's work experience by temporary stay in the US. So is the idea of it.
Not normally for temporary intracompany work experience.
Just like the WAT summer work visa, it's about 'cultural exchange'. You can bet most Polish, Slovak and people from Bulgaria, Romania or Russia didn't really care about cultural experience. It was about $$$. It was for me certainly, having saved 5 grand over summer that time (USD). But I did work my ass off for two months with crazy hours.
I suspect his employer doesn't bother with L1 purely because it's more lengthy and expensive route and the intended duration of stay doesn't correspond either. The person doing it should also have stayed with company for some time and be of importance to the US branch of the multinational, ie managerial position.
Using J1 should be workable. After all, they're going to have everything on paper, so if it's not liked by Consulate/Embassy, he wouldn't get the visa sorted.
I know of many cases when people sorted J1 visa only to work in hotel, but tweaked the 'training plan' to include different departments etc, but in the end worked as housekeeping, bar or waiting staff. That is, J1 as internship, not WAT 4 months only. So long it's done properly with paperwork, intentions may not be obvious.
EDIT: I can't see a reason his current employer (the company that wanted him there and has nothing to do with flight training, ie no conflict of interest) would mind him doing bit of flying on weekends. He wouldn't be stupid not to do the TSA, so all good. As for J1, technically the sponsor probably isn't the company itself, but one of the big agencies, ie Council Exchange, Intrax etc. I don't think they'd give a hoot so long he stays in the work he was authorised to go to.
If he left, he would get out of status and then it'd be a problem, doing flight training for PPL out of status. He wouldn't be able to change work on internship to FTO anyway.