Dear Chas:

The answer would be easy if this fella still thought of himself as Catholic. The answer would be certainly no if he was still Catholic marrying outside the Catholic Church. 

A Protestant baptized man and a Protestant baptized woman who marry have a valid and sacramental marriage. But, if a Catholic marries outside the Church without the bishop's dispensation, then the marriage is not valid, even if the wife is baptized.

In this case we have a Catholic man who has defected from the true Faith in favor of some non-denominational denomination (an ecclesial community, since this non-denomination and all Protestant groups are not valid "Churches" properly so-called).

More disturbing is that this defected Catholic, now Protestant, is intending a marriage ceremony in a restaurant and not in a "church" building? I am utterly amazed how disrespectfully people treat the holy estate of marriage by getting married in a balloon, underwater, on the beach, while sky-diving, before a justice-of-the-peace, and many other myriad ways, and now in a place of eats (how profane can people be).

Frankly, I would not attend the wedding just because this fella and his intended one have no class and prefer to sully the sanctity of marriage by having the ceremony in a restaurant of all places. How crude and profane.

It is possible to attend a wedding of a defected Catholic if that defection is complete, the person has no intention of going to Mass and receiving the Eucharist ever again, and when there is no reasonable expectation that this foolish soul will ever re-vert and come home.

This, however, is presuming the defected Catholic, now Protestant, is having a "church" wedding in their denominational "church" building.

I would even say that it may be possible to attend even if the wedding is in some neutral location, like a beach, lakeside, or even in a meeting hall at some hotel, but in a restaurant? A place where people are masticating? Does this couple think a marriage ceremony is just a party? Walk down the aisle while eating a chicken leg? For pete sake.

It is up to your conscience. You need to follow your conscience.

But, in my opinion, even if everything else is okay, I would not attend because of how this couple intends to be married. Their intentions for the ceremony show, I believe, that they are not ready for marriage. They are too immature and have no idea what marriage is about. If I were in this situation I would not attend the marriage because of the mockery they are making of the ceremony and because of the gross immaturity of this couple. But, that is me.

God Bless,
Bro. Ignatius Mary