How to convince a woman to allow me to lick her boots?

I hope I'm not inopportune, worse I feel that some girl in this forum could give me some good advice:

am a fetishist of boots, I love to see them and that my partner uses them in sex, which has led me to give them to my girlfriends. I have a friend for 12 years. I was always attracted but she was married and I am now. During these years I have given him several pairs of high boots (all platform and heel of 15 cm and more). Finally, I dared to tell her about the fetish, which she assumed very naturally (I suspected), because she is an open-minded woman and she loved those gifts. A few days ago I asked him to allow me a boots worship. He did not accept being married (my wife knows about the fetish, is not attracted to him and gave me freedom to exploit him as long as it does not mean having sex with another woman). How can I convince her?