Do you go down on her as enthusiastically and for as long as you used to?
Do you have an orgasm when she gives you a BJ? It's one thing if you just want something different, and another if you feel she's just not interested enough.
Have you tried *sincerely* complimenting the oral sex she performed on him? Words like, "That blow job you gave him the other night was amazing," are better than, "Why don't you blow me like that anymore?"