Kathleen Ann Harris, 39, of Olney, Md., previously pleaded guilty to the same charge in connection with the death.
Valentine said she discovered only after Eichelberger and Harris had been charged with her son's death that she knew Harris personally.
"She had been in my home," Valentine said.
Harris was the mother of girls who were friends of Valentine's daughters, she said.
Authorities found messages on Trey's phone.
"She called him two days after he had passed offering him more drugs. She didn't even know he died," Valentine said.
Harris is expected to be sentenced Dec. 19.
Holidays, especially Christmas, Mother's Day, and her son's birthday, are unbearable, Valentine read from her statement. She noted that she'll never get to attend his high school graduation, departure to boot camp or wedding.
Trey had planned to sign up for the Army, something she said she supported because he had been getting in trouble.
"He needed something," she said.
Ironically, she said in her statement, the thought of him in the military gave her pause. "I was afraid he wouldn't come home."
She said she had suspected some sort of drug use, and thought it was alcohol. Her son had been arrested for drunken driving in July 2006, Valentine said.
Valentine said that on Thursday she showed the judge a 6-inch stack of mail addressed to Trey that has arrived since his death.
Harris and Eichelberger started selling prescription medication in 2006. Harris had medical problems that allowed her to get prescriptions from doctors, according to information in the plea agreement.
Eichelberger and Harris sold drugs to Angle for several months, and some of the teenager's friends were present for some of the transactions, court records show.
On July 23, 2007, Trey purchased methadone from Harris and Eichelberger near a small shed in Boonsboro, according to court records.
On July 23 and 24, 2007, he used some of the methadone he bought from Harris and Eichelberger, the plea agreement said.
Methadone can stay in a user's system for up to 60 hours.
Harris and Eichelberger went to the teenager's home on July 25, 2007, where they again sold him methadone. He sent two of his friends out of the room while he consumed the methadone, and he and the friends consumed alcohol while there, according to the plea agreement.
He was found dead in his home the next morning.