In a weak attempt at further clarification (thanks to Google and awkward Google translations):
From age 15 on, it seems as though Henen was regarded as highly as any Belgian youngster. There was some acrimony over his move from Standard to Anderlecht, although that's not uncommon anywhere.
He had some issues with immaturity at Anderlecht (showing up late for training sessions, that sort of thing), and fell out of favor to an extent with management. Those habits caused some of his peers to advance ahead of him to the senior side, following which came the aforementioned disagreements over the alleged interest big clubs had in him.
He went to Monaco B for game time, but failed to find much, if any, due in part to Monaco's heavy investment. Games with Belgium's youth teams were his only outlet.
In an interview, Henen fessed up to his lack of professionalism at times, and claims to have at last learned what it takes to make it in football. No idea if
this link to the interview works.
That's the best picture of him I can create. Talented but immature. The worry of course is that his peers caught up with his talent, but he hasn't played much at the club level recently, so it's impossible to know. Hopefully his performances for Belgium's youth sides serve as proof that his ability is still real.