A non practicing jew (born from a jewish mother) is getting married to a non-jewish woman. They are having only a civil ceremony. Is it proper for a jewish person (a relative) to partake and wittnes such an event?

There might not be a clear Halachic problem attending the wedding, yet there is a prohibition to support a sinner performing his sin even if it will happen without you [Gittin 61a Rashi] how much more so if it can't happen without your help [Psachim 22b] On the other hand there is a Mitzvah to love your fellow Jew - that includes your relative as well, how much more so if by keeping in touch you [by attending the wedding] you increase the chances of him returning to his Jewish roots [and divorcing or converting his wife]. Attending such a wedding will normally be understood by the groom and family as an approval to the match; more so your absence from the wedding will be a strong statement that you can't attend an event of a Jew turning his back on his people and heritage. I will recommend sending a letter explaining your dilemma and your decision based on the conflict mentioned above. If the case really is such that your attendance is crucial for maintaining the only thread that may bring him back one day and by not attending he will turn his back on you and therefore his Judaism for ever and you being at the wedding is not looked upon as an approval – there is an argument to allow your attendance. In such case you should remind yourself you are witnessing a tragic moment of a Jew cutting himself from his roots rather than a joyous family celebration. Eating at the event is a separate issue which brings up more Halachic challenges; I will recommend you to have your dinner at home. [see SA YD 152:1]