Because they are reportedly going to co-parent their son, the source believes "it's highly unlikely this is going to get ugly."
Since getting together in 2011, the "Hello" superstar and her husband, a 44-year-old philanthropist, kept their relationship extremely private and out of the public eye. However, during Adele's musical events, Simon was always by her side.
"Their marriage was clearly working for a very long time," the insider adds. "Whenever she was out at big openings and special events, he was usually with her, and they looked really just happy together and in love."
As the source puts it, "They were into the marriage pretty much until they decided this just isn't working anymore."
The news of the pair's split comes just a month after the 30-year-old songstress was photographed without her wedding ring. She was headed into a recording studio in New York City.
Furthermore, in the beginning of the year, she had a girls' night out with gal pal, Jennifer Lawrence, where the two hit up a few gay bars in the Big Apple. The "Skyfall" singer also enjoyed a night out with Drake at a Los Angeles bowling alley in late February.