The 35-year-old Scrubs star reportedly began dating Cobb, 31, in 2006.
Their romance followed shortly after her stint on Simpson's reality TV show Newlyweds, which made a spectacle of the singer's marriage to ex-husband Nick Lachey.
According to Star magazine, Simpson's best pal and former co-star will soon tie the knot.
A source says, "She's told all her girlfriends. Donald's looking for an engagement ring, and they've talked about the bridesmaids, having kids, everything. Jessica's been really getting into planning all the wedding details."
A spokesperson for Faison had not responded to requests for a comment as WENN went to press.