"Ashlee loves John," a Simpson pal tells PEOPLE in its new issue. "She thinks he is great for her. More importantly, Jess is happy."
Ashlee isn't the only one in a positive mood. "John seems very smitten," says another Simpson pal.
Just back from a romantic trip to Rome, Mayer and Simpson have been making the amore last back in L.A., with kisses over lunch, nuzzles at a James Morrison concert and hand-in-hand strolls.
On March 20, Simpson, 26, and Mayer, 29, happily snapped pictures with fans at a gig of Mayer's bandmate David Ryan Harris at the Hotel Cafe. And catching Morrison's show at the Roxy, "They were smooching throughout," says a fellow concertgoer.
Afterwards, over cocktails at Parc restaurant with Ashlee, 22, "They were holding hands and laughing constantly," says an onlooker. "Jessica looks really comfortable with him."
Friends say Simpson is relishing this romance, which is low-key compared to her time with ex-husband Nick Lachey.
Observes another pal, Christina Milian, who was in Simpson's "Public Affair" video: "Clearly this is a relationship that's not for the cameras. It's not about celebrity. It seems like they truly have an attraction to each other, and they're both clearly very talented as well."
Reporting by KC BAKER, JENNIFER GARCIA, LISA INGRASSIA AND KIMBERLY LANSING