When Missy Armstrong learnt that her best friend was dying, she knew there was one thing she could do that would help to ease her friend's fears - adopting her four children.

Sara Hankins, 36, was diagnosed in 2015 with amyotrophic lateral sclerosis, also known as Lou Gehrig's disease, a neurological disease, and is currently in hospice care.

Her biggest fear was what would happen to her children after she passed away - until her friend of 20 years Ms Armstrong stepped up with her selfless offer.

"I thought, yeah, I can do this. I love them, and I've known them their whole lives. I make a decent living. I have the support of her family," mother-of-two Armstrong, 42, of Illinois in the US, told NBC's Today show on Dec 17.

The two single mums became best friends after meeting during training to become hairdressers in 1999. Armstrong now works as an equipment operator for a metals manufacturing company.

"She would grab her kids and come to my house, or I would grab my kids and go to her house. We were always together," Armstrong said.

Armstrong is currently in the process of legally adopting three of Hankins' four children, aged eight, nine and 11. She will also take in the fourth child, who is 18.

Hankins told of her shock at Armstrong's generous offer.

"I couldn't stop crying. My number one concern was keeping my kids together, because I think what they're going to go through is hard enough," she told the Today show.

As for Hankins' children, Armstrong told Today they have "an idea" of what's happening.

"Of course it's not going to hit them until it hits them," she said. "But everyone has been honest with them."

And they already consider Armstrong family. "Me taking them is the way they can all be together," she said. "And that's of course what Sara wants. We've been best friends and we love each other. And I love the kids."

Now, loved ones are making sure Armstrong has a big enough house to hold her new family. A Go Fund Me page was started to help raise money.

Hankins' sister said that one of Sara's last wishes is to secure a home for Armstrong and her suddenly large family, said Fox 8.

"Although Sara won't be the one raising her children, and teaching them life's lessons, she finds great comfort knowing they will be loved and have their own 'Place to Call Home,'" wrote Hankins' sister.

The family is over halfway towards its US$20,000 (S$29,000) goal at US$12,000.