“Epis a friend i have who nebach wants to have mixed dancing at his chasuna. How can i convince him not to ??? “
Assuming this post is not a joke (which sorry to say it really sounds like a leg-puller to me), you cannot convince your friend of anything. It is his wedding, his family dynamic, and his business. You can, if you are truly understandably uncomfortable, attend the chuppah only, and if he asks why not the whole wedding, tell him that while you are very happy for his getting married and wish him mazel tov, that you don’t attend simchas where there will be mixed dancing. You cannot change what his family wants to do at their simcha (or the other side might be extremely modern and wants mixed dancing), but you can act according to your own conscience. If enough of his friends tell him they cannot attend the entire chasunah for that reason, perhaps he will try to change the plan for the mixed dancing.
BTW, were this my own situation, I would simply refrain from dancing. I would personally not seek to hurt someone’s feelings at his own simcha. For the sake of Sholom Bayis (with extended family members), I have attended my husband’s family simchas where the FOOD was not kosher, let alone the dancing! My husband and I wished the families involved mazel tov, and then sat quietly. And when asked to dance OR eat, we simply said thanks, that we would love to, everything looks delicious, but that we cannot, and that we do not dance in mixed groups. No big explanations, no pontifications. My in-laws truly appreciated our attendance, nonetheless. And in the eyes of the frei Jews who were there, we were menschlech (meaning, they didn’t think we were stuck up holier than thou-ers, and they respected us for it.