Inside Matt Damon's Crucial Support of Pal Ben Affleck Amid His Sobriety Struggles (Exclusive)

Matt Damon and Ben Affleck at Live by Night premiere
Frazer Harrison/Getty Images

The source added that Garner is "so appreciative" of Damon's support for her ex.

Having known each other for an incredible 37 years, Matt Damon and Ben Affleck remain one of Hollywood’s favorite bromances.

Now, as Affleck continues to fight to stay sober, having entered rehab on Aug. 22, ET has learned that Damon remains a key supporter in his recovery.

Despite the fact that the two had a blast partying together back in the day, a source says Damon stopped drinking with his pal long ago, to avoid enabling him.

"When Matt and Ben were young, they loved to party together and have fun, but as soon as Matt realized Ben had issues with substance abuse, he made sure he wouldn't enable him,” a source tells ET. "Matt stopped drinking with Ben because in no way does he want to support another downward spiral. Matt is a very loyal, stable and grounded friend who only wants success and happiness for Ben.” 

The source added that despite the two maintaining a close friendship over the years, Damon was taken aback to learn about the seriousness of Affleck’s current struggles.

"Matt and Ben have always remained close friends in life, but when Ben relapsed, it was a surprise to Matt,” the source said. “Matt had no idea how serious Ben's problem had become.”

"Matt wants to be a good influence on Ben,” the source added. “Ben's team, his closest friends and family truly support their friendship. They want them to hang out more, which in turn could help keep him away from the crowd that doesn't support his sober lifestyle.”

As for Affleck’s ex-wife and the mother of his three children, Jennifer Garner, she’s said to be all for Damon’s help.

"Jen is so appreciative that Matt is giving him positive encouragement and advice,” the source says. “Ben trusts Matt and Jen knows Ben needs those closest to him to get him through this. Ben has truly struggled with his sobriety and hopefully with rehab, the support of Matt and his family, he will finally be able to remain sober and live a healthy lifestyle.”

ET talked to Affleck’s brother, Casey, at Monday’s premiere of his new film, The Old Man and the Gun, in Toronto, Canada, where he gave an update on the 46-year-old Oscar winner.

"He’s doing great," Casey, 43, shared. "He’s in rehab and he’s recovering and seems to be doing well."

See more on Affleck and his struggles below.

*Reporting by: Adriane Schwartz