Claire Danes and Mamie Gummer
Peter Kramer/AP; Jamie McCarthy/WireImage
Not even the stress of wedding planning can cause Claire Danes to lose her cool, a friend of the star tells PEOPLE.
"She's so calm and excited about it," Mamie Gummer says of the actress' upcoming nuptials to Hugh Dancy. "She is going to be the most beautiful bride."
Gummer costarred with Danes, 30, and Dancy, 33, in Evening
, which was filmed in Newport, R.I., in 2007. She says it was privilege to watch the pair meet and fall in love as they bonded over games of Boggle and Scrabble.
"I was there when it was happening and it's something that I will never forget," Gummer tells PEOPLE at the Chanel and Tribeca Film Festival Artists' Dinner in New York on Thursday.
Gummer is also a close friend to the couple, who got engaged in February
, and will take part in the wedding ceremony.
"I'm going to be a bridesmaid," she says. "I am so honored that she asked me and I'm incredibly happy for them. They are perfect for each other." As for the date of the wedding, "I can't tell you that," Gummer replies coyly.