He was not Catholic only she was, however shrews married outside oof the church the second time I believe which is not in good standing
with the church.
Oops “she was” and outside of the church. Sorry
I remember reading that the Orthodox Church allowed people who had been divorced once to be married in a church wedding (I don't know if that is specific to the Greek Orthodox or applies to all of the Orthodox Churches). The problem for her as a Catholic would be that the Church would not recognize her marriage to a divorced person as valid (any more than Henry VIII's marriage to Anne Boleyn). That Onassis was not a Catholic would be of minor importance--the Church wants Catholics to marry other Catholics but they perform lots of weddings where the other partner is a Protestant, and being Orthodox should be a lesser hurdle than being a Protestant.