Looks like a tabloid bull**** story. Make something up but include a famous name and pick up your cheque. Why would Boonen sell drugs, he's worth millions. His name alone is a huge gravy train so why drive it staight off the track for a few quid. Any money he could make with a little bit of dealing is less than he'd make from one appearance in Belgium on TV or at a race.
That is one problem with the drugs issue in cycling becoming high profile, you are bound to get a load of "shag n' tell" type stories from people trying to get rich quick.