The teenager's act of kindness has earned her praise from her fellow passengers, the airline and her mother Jane, who was travelling with her daughter.
But no praise for Daly has been more glowing than that from the man she helped.
Speaking to Portland TV station KGW8, Cook simply said: "She's an angel".
Lynette Scribner, who was seated next to Cook, took to Facebook just moments after stepping off the plane to tell the amazing story.
Her post and photos of Daly signing into the palm of Cook's hand mid-flight have now been liked almost 950,000 times.
"I saw this man, Tim, in Boston's Logan airport with the sister he'd been visiting... it appeared he was both deaf and blind," Scribner posted.
"The attendants sincerely wanted to assist him, but had no way to communicate. I watched as they didn't flinch when he reached out to touch their faces and arms.
"That's when this lovely young woman came into the picture. 15 years old, she learned ASL because she had dyslexia and it was the easiest foreign language for her to learn.
"It was fascinating to watch as she signed one letter at a time into his hand.
"When he asked if she was pretty she blushed and laughed as the seat mate, who had learned a few signs, communicated an enthusiastic 'yes' to Tim.
"For the rest of the flight she attended to Tim and made sure his needs were met."
Daly said she and Cook spent much of the flight just chatting like friends, and gave each other a hug as they departed the plane.
"For the last hour of the flight we just were talking, having a conversation about life," the teenager said.
"They call you extraordinary but it's just something that you do. He took such joy in a conversation that we take for granted."