Henry, a New Zealand Boxing selector, makes an annual trip to the United States to touch base with his jujitsu and boxing contacts.
Two years ago, he met boxing great Floyd Mayweather which was a thrill. But this trip Parker's coach, Kevin Barry, invited Henry to attend a private training session at a gym in Las Vegas.
''I thought that we might just be catching up but he invited us to a private training,'' Henry said.
''We sat there for a few hours just watching the whole thing.''
Parker was preparing for his WBO heavyweight title defence against Brit Hughie Fury, but the fight fell just over two weeks before the scheduled bout with Fury citing a back injury as the reason for pulling out.
Romanian Razvan Cojanu stepped up at short notice and the 30-year-old will fight Parker in Auckland on Saturday night.
But Cojanu does not enjoy the profile Fury does, so the bout represents a loss of momentum for the Parker camp.
Even a decisive victory will probably be shrugged off. And that big fight Parker desires is just that little bit further away, which is a shame because the sport is really buzzing following Anthony Joshua's recent 11th-round victory against boxing great Wladimir Klitschko, who dominated the division for a decade.
But Henry does not see it as a huge negative for the 25-year-old Parker.
''Because he is so young it is possibly not a bad thing. He still has plenty of time because I don't think you mature as a heavyweight until later on anyway.''
Much has been made of Parker's power and whether he has the punch to knock down some of the more durable boxers in the division.
While he boasts a professional record of 22-0 with 18 knockouts, he was not able to stop Carlos Takam or Andy Ruiz and that has been offered as evidence by some observers that he lacks oomph.
But after seeing Parker close up, Henry has no concerns on that score. However, it was his speed which impressed the most.
''I wouldn't want to be hit by him, that's for sure,'' he said.
''When he is on the pads and going through set routines, that speed of his was not interrupted ... and it was phenomenal.''
Henry believes Parker will be too quick for Cojanu but still thinks it is a potentially tricky match-up all the same.
The Romanian has a record of 16 wins and two losses. Nine of his wins came by knockout, so he is capable of sending Parker to the canvas if he can catch up with him.
He will have a useful height advantage at 2.02m compared with the 1.93m Parker. And the boxers know each other well. They sparred against each other while Parker was preparing for the called-off fight against Fury.
''That is probably going to go in Joseph's favour, especially seeing how confident he is.''
