Is it forbidden to hold an honoring banquet for members of scientific research laboratories in Western countries after the defense of PhD theses, given that it involves the mixing of men and disbelieving women?

If it is possible to hold the banquet without the mixing of men and women, then do so. If it is not possible except with mixing, then it is forbidden to hold it, as it would be an assistance to evil and an acceptance of it. This is based on the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty." However, if you are compelled to do so and significant harm, recognized by Islamic law, would result otherwise, then there is no blame in holding it due to necessity, which is assessed according to its extent.