Inviation to Wedding of "Mixed marriage"

Someone (a Bal Teshuva) received an invitation to the wedding of a close relative. Unfortunately the wedding is with Jewish man and a non-Jewish woman. If the relative doesn't respond the one getting married will certainly feel hurt and understand something is going on. On the other hand perhaps there is a problem of saying to him "Mazel Tov, B'Shah Tovah U'Matzlachas!" What is the best thing to do and or say in this situation?

This question was posed to many Rabbonim and came with a variety of answers. I was surprised to some extent that Rabbonim said that no response was given and simply to cut off the connection that these 2 relatives have with one another. The most response I heard that made the most sense was that the frum relative should wish the not frum one, "You should merit to build a Jewish Home!". This way somewhat avoiding the difficult issue of approving the marriage and at the same time, giving a nice blessing to this person with somewhat of a hint as to what he really feels. If anyone has what to share on this, please do so and leave a comment!