His Glee costar "is an excellent influence on him," a Monteith source tells PEOPLE
Credit: Dimitrios Kambouris/Wireimage

As Cory Monteith continues his stint in rehab, friends say he has a stabilizing element waiting for him when he returns home to girlfriend Lea Michele.

“Lea is an excellent influence on him. She’s good for him,” a Monteith source tells PEOPLE. “She’s really calmed him.”

Since taking their relationship public in April of last year, Monteith, 30, and Michele, 26, appear to have been a positive influence on each other.

“They seem to compliment each other’s personalities very well,” says another source. “They are always very affectionate and sweet to each other. Cory acts very protective of Lea.”

While friends say the Glee costars are “low-key” when out and about, they did not live a completely sober life.

“The two of them drank together, but drank in a way that seemed to be responsible,” says a source in the couple’s social circle. “They both seemed in control and not messy.”

It came as no surprise to those close to the two that Michele released a statement to PEOPLE supporting Monteith’s decision to seek treatment.

“She would have been very supportive and encouraging for him to seek treatment,” the Monteith source says. “It seems like she is a very steady hand.”

Additional reporting by PERNILLA CEDENHEIM and DAHVI SHIRA