The 38-year-old actor attended the Kind Los Angeles: Coming Together for Children Alone event in Culver City, California, on Saturday night.
Wearing a light blue Oxford shirt, slim black pants, brown loafers and gold and silver jewelry, Valderrama looked dapper for the event, which was helping to support immigrant children by providing them with pro bono lawyers in deportation proceedings.
"They have mutual admiration for each other and he has and will always believe in her. He's watched her fall and has seen her rise," a source told ET. "[Wilmer] was aware she was going through a bumpy patch because she was forthright with him when she relapsed. But he did not anticipate this."
"Demi is getting better day-by-day with the help of her family, friends and Wilmer," a source told ET. "He visits Demi every day he can, and has spent hours with her and her family."
Also over the weekend, Lovato spoke out for the first time since her hospitalization. "I have always been transparent about my journey with addiction," she shared in an Instagram post on Sunday. "What I've learned is that this illness is not something that disappears or fades with time. It is something I must continue to overcome and have not done yet."
"I want to thank God for keeping me alive and well... I now need time to heal and focus on my sobriety and road to recovery," she continued. "The love you have all shown me will never be forgotten and I look forward to the day where I can say I came out on the other side. I will keep fighting."
A source told ET last week that the "Sorry Not Sorry" singer is "doing much better" and is expected to leave the Cedars-Sinai Medical Center in Los Angeles very soon.