The "American Idol" alum flew from London to New York to attend the Breast Cancer Research Foundation’s Hot Pink Party with her husband-to-be. McPhee is currently starring in "Waitress" in the city's West End.
McPhee, 35, took the opportunity to gush over her fiance, Foster, 69, before the pair performed together at the event.
"We’re so happy and proud of each other," McPhee told People. "I believe in love."
The singer, who was previously married to director Nick Cokas, also doled out advice to her single fans. Foster has also walked down the aisle -- four times -- most recently to "Real Housewives" star Yolanda Hadid.
"It’s going to happen when you’re not looking for it. It’s going to be the person that’s right under your nose, or it’s going to be somebody you’ve known for a long time, or it’s going to happen when you least expect it. It really is cliché but it’s cliché for a reason — because it’s true! It certainly happened for me that way," she advised.
The singer often shares public displays of affection towards her fiance. In February, McPhee posted a sultry photo of herself directed at Foster.
"Far away from each other but still can't wait to marry you," She on her Instagram Story while tagging Foster. The singer also casually showed off her massive engagement ring in the post.
The music producer got the sweet message and reposted it on his Instagram writing, "I won the lottery!"