The Spanish fourth seed is into round three after outclassing Dutchman Robin Haase 6-1 6-4 6-3 on Wednesday.
The 30-year-old would become the first player in the Open era to win 10 titles at one Grand Slam, should he progress.
"It would be such an incredible achievement by such an incredible player," said Henman.
"You can't even fathom it. A lot of people don't win 10 tournaments, let alone 10 Grand Slams at one venue," added the Englishman.
Nadal, who is 31 on Saturday, won his first Roland Garros crown in 2005, the first of four successive victories before claiming a further five in a row between 2010 and 2014.
The Spaniard is only one of two players - the other being German Alexander Zverev - to win multiple clay-court titles this season. He won his 10th titles at Monte Carlo and Barcelona as well as his 30th Masters title in Madrid.
"It is the biggest challenge in the game to beat Nadal over five sets on clay in Paris," Henman told BBC Sport.
The only other player to win 10 or more titles at a single Grand Slam is Margaret Court, who won 11 Australian Opens between 1960 and 1973.
"It would be one of the biggest achievements in tennis and one of the biggest in any sport," added Henman.
"To have been so dominant at one of the biggest events in our sport is just incredible."
After a season hampered by injuries and illness, world number one Andy Murray beat Russian Andrey Kuznetsov in the first round on Tuesday and Henman believes the Scot will be looking to "build round by round".
"I wouldn't say Murray is favourite but he is the number one in the world for a reason," Henman said.
"I haven't been concerned by his form because he has been ill and injured.
"If he can stay healthy and injury free, he will start winning matches and tournaments because he is one of the best players out there."
The Scot won his opener in four sets to set up a second-round match with Slovak Martin Klizan. He now faces a potential third-round clash with Argentina's Juan Martin del Potro.
"Klizan is an awkward player but I expect Andy to come through that and Del Potro in the third round is a much bigger challenge.
"Irrespective of what happens in Paris, on the grass, with his record at Queen's and Wimbledon, he will definitely be one of the favourites for those titles."
Former world number four Henman believes Andre Agassi will have "lots to offer" Novak Djokovic, after the American took over as the Serb's coach for the first week in Paris.
Djokovic hired eight-time Grand Slam champion Agassi, having parted company with his coaching team earlier in May.
"Agassi is massively experienced and went through a lot of highs and lows," Henman said.
"He is not going to teach Djokovic to hit new shots but can help him use his attributes to the best of his ability.
"Agassi's experience and a new voice can certainly help. It can sometimes be someone saying the same thing but in a different fashion."

Firefighters were called to the Toys R Us store in Kingsway West Retail Park at 00:34.
Seven appliances including an aerial unit tackled the fire in an external canopy at the rear of the building.
The Scottish Fire and Rescue Service said the store was heavily smoke-logged as a result, but the fire was extinguished within two hours.
A Police Scotland spokeswoman said officers were keen to trace a person seen in the area at the time who was wearing a light-coloured hooded top and dark-coloured trousers or jogging bottoms.
Rafael Nadal winning a 10th French Open would be "one of sport's biggest achievements", says former British number one Tim Henman.
[NEXT_CONCEPT]
Police in Dundee are investigating whether a major fire at a Dundee toy shop was started deliberately.