Lt. Steve Tenney went above and beyond the call of duty last year when donated part of his liver to an ailing baby girl

It was a parent’s worst nightmare. Sloan St. James was born a healthy, bouncing baby in April. But her parents, Sarah and Chris St. James, knew there was something wrong when their baby girl remained jaundiced for months after her birth.

“I brought it up to our pediatrician and they weren’t concerned about it. But [Sloan] started getting a little belly and her belly was getting more swollen than the rest of her,” Sarah, 33, tells PEOPLE, adding that this prompted her and Chris, 33, to take Sloan for another check-up. “I wanted to have peace of mind that everything was okay. But when I took her in on August 8, the doctor that saw her was very concerned.”

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Doctors soon discovered that Sloan has biliary atresia, a rare, life-threatening liver disease in which the baby’s common bile ducts are blocked or damaged.

Image zoom Courtesy Sarah St James

“It was the worst time of my entire life. It was the scariest thing I’ve ever encountered,” Sarah says of the diagnosis. “The very first thing we found out was that her liver was in stage 4 liver failure, which is the final. It’s the end. It was absolutely terrifying.”

The St. James’ learned that the infant would need a liver transplant. So, they took the baby back to their Buzzards Bay, Massachusetts, home and waited for Sloan to match with a deceased donor. But Sarah says Sloan’s health was declining too quickly to wait.

“Her liver was just failing over time. We thought we could wait it out until we got a match, but she started failing even quicker,” Sarah tells PEOPLE. “She was just getting sicker and sicker. We couldn’t just wait around for a deceased person to match her.”

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Throughout their ordeal, Sarah had been sharing updates about Sloan’s health on Facebook. And one caught the attention of Lt. Steve Tenney, of Keene, New Hampshire, who the couple knew through mutual friends. He eagerly offered to help the family by donating part of his liver to the little girl — as it turned out, he was a perfect match.

“It was incredible,” Sarah says. “It’s rare that the very first person is a perfect match.”

Image zoom Sarah St. James (center) with son, Carter, and daughter Sloan Courtesy Sarah St James

On Sept. 8, Tenney, 40, donated a small piece of his liver to then-4-month-old Sloan at Lahey Hospital & Medical Center in Burlington, Massachusetts — in a decision he says was a no-brainer. (Sloan underwent the transplant at Boston Children’s Hospital.)

“If you can help a suffering child you will,” Tenney, of the Keene Police Department, tells PEOPLE. “I think most people would if they were in a position to help any child in need. It’s kind of a personal thing that everybody should have that humanity that if they were in a position to help someone that they could and should.”

Just a few weeks later, Sloan and Tenney met for the first time at the Boston hospital, where he hugged the smiling baby.

“She’s an adorable little girl. It’s great to see that although she still has a road to recovery that she’s doing good and has a chance at living a normal life,” he tells PEOPLE. “She’s a very happy baby and very adorable.”

Tenney says the whole ordeal has bought the two families together, noting that his children, aged 10 and 18, and wife, Aubrey, encouraged him to donate,

“I will, even from a distance, keep an eye and make sure [Sloan’s] doing okay,” he tells PEOPLE.

As for Sarah, the mom of two says she hopes to visit the Tenneys and plan a celebration for the one-year anniversary of the surgery.

“We feel like a new bond is created and we just love that,” she tells PEOPLE. “We’ll forever be connected our entire lives which is really exciting.”