Is it morally acceptable for a Catholic to attend a wedding between two Jehovah Witnesses, one of whom is an ex-Catholic?
If “ex-Catholic” means that the party has formally defected from the Church, then that party may not be bound by canonical form and the wedding may be presumed valid. “[F]orm… must be observed if at least one of the parties contracting marriage was baptized in the Catholic Church or received into it and has not defected from it by a formal act.” (Code of Canon Law, 1117) If this is not the case, the ex-Catholic may be bound by canonical form and a dispensation would be required to validly marry outside the Church.
The Church does not explicitly disallow attending an invalid wedding. However, I recommend reading Should I Attend? for guidance on this topic.