EVERETT, Wash. -- A babysitter pleaded guilty on Monday to beating his friends' 1-year-old son so badly that the baby may have permanent brain damage.

Matthew Christiansen admitted to having hurt Eryk Woodruff last September while he was watching the baby along with his 3-year-old sibling.

Last year Eryk was first admitted to Harborview Medical Center, then transported to an assisted living facility in Woodinville after doctors found several broken bones, including a broken jaw and rib, as well as a serious brain injury from the assault.

In addition to the broken bones and head injuries discovered at Harborview, doctors there found older bruises on the Eryk's body, and investigators determined that Christensen had taken care of the boy three other times prior to the latest incident.

Eryk has since moved home, but he now has to wear a helmet to protect his brain.

In past months Eryk has had spurts of improvements that have been overshadowed by unexpected setbacks. Doctors don't know whether the boy will ever be able to function at the full capacity of his age.

Monday was the first time the boy's parents faced Christiansen since the baby's struggles began. Eryk's mother Rachel Pierce said she has been waiting for months to hear her former friend own up to his mistakes.

"It will always be hard to think about the fact that we let those people (Christiansen and his family) into our lives. They were our family's best friends. And if you can't trust your best friends, who can you trust?" said Rachel.

Christiansen could face up to ten years in prison.

Friends have set up a fund to help Eryk's family. You can donate to the Eryk Woodruff fund at the Washington Mutual Branch in Mill Creek.

"From the original surgery, he got a staph infection. He has no bones on the right side of his head," his mother Rachel Pierce said.