The American tourist who was devoured by a shark in Indonesia is believed to have died from a “medical issue” while diving, her friends told The Post Monday — revealing that she was an experienced deep-sea diver who was “really enjoying life.”

Colleen Monfore, a retired mom of two from Holland, Michigan, was enjoying her dream vacation — a seven-week diving tour with her husband Mike — when tragedy struck on Sept. 26, family pal Rick Sass said.

Just a few days into the trip, Monfore, 68, disappeared during a group dive around Pulau Reong island, north of Timor-Leste and off the coast of the Southwest Maluku Regency. Two weeks later, fishermen in Timor-Leste caught the shark — and cut human remains out of its belly more than 70 miles from where she vanished.

Authorities were able to ID Monfore from the body’s fingerprints, according to Sass, a longtime friend who has been in close contact with the family throughout their harrowing ordeal.

“We do not believe this was a shark attack. Mike thinks she suffered some kind of medical issue in the water,” Sass told The Post.

Sass and his wife Kim – who ran a dive shop together for more than 40 years – examined photos from the dive, spoke with Mike at length about the incident and reviewed data from his dive computer.

The friends are adamant Monfore, who Rick Sass said seems to have been separated from the group when rough waters forced them to turn around, was not killed by a shark. She was around 24 feet down and probably had half a tank of air left, he said.

“There was a down current at the turnaround site, but it was manageable,” Kim Sass wrote in a Facebook post. “I’ve easily done 1000+ dive[s] with this gracious woman … I don’t believe it was the environment and certainly not a shark that ended her life.”

Rick Sass added: “They dove massive numbers of trips with us over 30 years. Bali, Philippines, Bikini Atoll, you name it.

“She knew what she was doing.”

Mike Monfore was already an avid diver when he met Colleen in high school in their native state of South Dakota, and she soon too fell in love with the ocean.

They had two kids and, eventually, four grandkids, but they never stopped traveling the world and exploring the seas together. “They were both retired and really enjoying life,” Rick Sass said.

“We used to call her ‘Saint Colleen.’ She was an amazing woman,” he added. “She loved nature and animals. I know she would have never wanted a shark to be blamed for this tragedy.”

He noted that shark attacks against divers are extremely rare. 

“And sharks are not that way. We’ve dove with hammerheads, tiger sharks, bull sharks. You have to give them respect and be cautious, but they won’t just attack you.”

There were only 69 confirmed unprovoked shark bites worldwide last year, according to the Florida Museum of Natural History’s International Shark Attack File, and 94% of those were on snorkelers, waders, and surfers. The remaining 6% are categorized as “other.”

It’s not yet clear how exactly Colleen Monfore died. Grisly photos from the scene show the shark in question cut open — with human remains in a black wetsuit nearby.

“The shark was caught but it was not in normal health. I thought it had swallowed plastic or a fishing net,” the fisherman said, according to Asia Pacific Press. “It was cut open to find the problem and inside there were the remains of a woman.”

Rick Sass said her husband has been struggling not only with her death but bringing his wife’s remains back home.

“He’s hardly been sleeping at all, partly because he’s up on the phone with Indonesia, which is a 12-hour time difference,” he said. “He’s answering phone calls in the middle of the night, and talking to us during the day, and not just not being able to sleep because of this horrible thing that happened.”

Indonesian authorities said an investigation is underway.