Coming to inter-caste marriages, there is nothing wrong with two mature adults get married. It is more important for the spouse or the families NOT to try and force the other to get converted to their caste, like, a boy from one caste or his family forcing a girl from another caste to get converted after the marriage (can be vice versa with other castes). Any grown up man or woman should be free to choose what caste or faith he or she wants to follow.
As far as your relative is concerned, I'm happy that he retains his identity. Getting married to a person of an other caste, following a spiritual leader of an other caste, spending time in social service related to a temple or a Church should not mean that he/she changes religion, doesn't mean that one should give up his faith and follow a new one. Do it by choice, do it once the decision and the likely implications are understood and accepted.
Caste system was devised by ancient people to distribute the work in society to different groups and these groups were not allowed to mix with each other. This was probably the handiwork of some intelligent people who did not like to do menial things by themselves and wanted to get it done by inferior castes.
Whatever be the motive behind such an act, today it has created a big political drama as well as leaders are exploiting the people and their sentiments on the basis of caste related matters. They are time and again reminding the lower castes how the upper castes exploited them in past and they should if possible now settle the score. This is in fact fueling the caste related fights.
The reservation system which helped the lower castes to get the job has now become a thing of great controversy in our country. People have become more concerned about caste related matters now a days. There is a great threat to the upper castes if the reservation system continues.
Knowledge is power.