"He's great, he's great. He's got a big smile on his face," Damon told reporters of Affleck, according to People magazine.
Affleck didn't speak to the media, but he did sign a few autographs, People adds. Photos also show the A-list actor and director was all smiles at the event.
Meanwhile, Damon didn't pull any punches when talking about Affleck's upcoming 43rd birthday later this week.
"It's kind of like, meh. What is forty-three? It's not forty ... It's a forgettable birthday," Damon joked.
Affleck and Garner, who are parents to Violet, 9, Seraphina, 6, and Samuel, 3, released a statement at the end of June, saying, "After much thought and careful consideration, we have made the difficult decision to divorce. We go forward with love and friendship for one another and a commitment to co-parenting our children."