"We've all been leaning on each other. It's been a rough patch and a rough time," Harry Shum Jr. told PEOPLE at Saturday's Invisible Children Founders Circle Party in Los Angeles.
"I don't know when that will ever end, that rough patch of losing a friend like that. It's so unexpected, but we're all leaning on each other."
Shortly after Monteith's passing, Shum posted a collage of photos on Instagram, paying tribute to his late costar. He captioned it, "This is how I remember him. Kind hearted. Incredibly funny. Extremely passionate."
"You look through old photos, if you lose anybody, you look at old photos and something comes up with those memories, and I wanted people to know – and I think they already knew – that Cory was just a great guy, an overall great person," he said. "Caring, loving, talented."
As for Lea Michele, Monteith's girlfriend and costar, Shum says he's been in touch with the grieving actress.
"Of course she's heartbroken, everyone is. But I think it's great that everyone is back at work and at least following through, and hopefully we can at least just get through this rough patch."
Fans of Monteith may make donations in his name to three charities that were especially important to him: Project Limelight Society, which exposes youth living in Vancouver's Downtown Eastside to the arts; Virgin Unite, founded to support entrepreneurial efforts to better the world; and