There are Tons of ways to go - Embroidery, Calligraphy, Applique, even paper piecing. What did you have in mind? How big? Do you want letters on separate blocks or some kind of banner? Do you want to spell out "Love" or use a heart? Will the writing cover the entire quilt, or be in a center medallion or border strip?
Before you begin a commissioned piece for someone, you need to be clear about what they are expecting. It will save a whoole lot of misunderstandings later on.