The mother of a missing 16-year-old Pennsylvania girl is pleading for her safe return. Police believe Amy Yu ran off one week ago with Kevin Esterly, the father of one of her friends.

The 45-year-old is allegedly in a relationship with the high school student.

Yu was last seen by her family at a school bus stop in Allentown, where she'd been dropped off along with her brother.

She has not been heard from again.

Amy Yu, 16, and Kevin Esterly, 45. Allentown Police Department

Yu's mother told CBS News' Meg Oliver that her daughter was taken by a man who befriended their family at church -- someone she had trusted.

Esterly and Yu had gotten so close she often went on his family's vacations.

On his Facebook page, Esterly often posted pictures of Yu hanging out with his own daughters. But police say their relationship was not as innocent as it appears.

Yu's mother, Mui Luu, says she feels betrayed. She'd had Esterly over to her house for meals.

Luu found out on February 9 that Amy had listed Esterly as her stepfather on school records. According to a police affidavit, Esterly signed Amy out of school 10 times between November and February without her mother's permission.

Luu says she also found text messages that suggested Esterly was romantically involved with her daughter, who is 16 -- the age of consent in Pennsylvania.

John Yu, Amy's younger brother, last saw her at the bus stop. He told Oliver he was upset: "Why would a 45-year-old man and a 16-year-old girl … like, that's disgusting!"

Esterly and Yu vanished on March 5. Esterly's wife told investigators he withdrew about $4,000 from his wife's bank account that same day. Police suspect the two are traveling together in Esterly's red Honda Accord with Pennsylvania plates.

Oliver asked Mui Luu, "What do you want to tell Amy?"

"I want to tell her, 'Amy, can you come back to me? I love you. I just hope you come back.'"

Prosecutors have charged Esterly with interference of custody of children. Amy's charter school has also banned him from school grounds since last month. "CBS This Morning" tried to get an update on the investigation from the Allentown Police Department, but no one was able to speak to us on camera.