Should I have with her daughter?

Hello, first let me say that I am very serious and feel free to email me. But I have a good friend who is a cougar and single and we have casual since her husband died 3 years ago. Well her step daughter and her are very VERY close and talk about -everything- and I am also friends with her. Well the daughter recently turned 18 and is still virgin. Well they recently brought up the idea of me taking her virginity because they trust me and know I would be gentle. They also stressed that it would be more for learning then pleasure. I am still weary though... What should I do? Please good words of advice and no stupid simple answers. Thanks -Mike