So I've been invited to an Indult wedding and mass, and I'm just looking for guidance/info to help make a decision on attending/not attending.
-I would consider the priests doubtful, as they are Novus Ordo priests, so reception of the Sacraments is a clear no.
-The groom is a friend, so I want to do what is charitable without transgressing any Church law/teaching.
Is there anything else I should consider, ie Church law on attending doubtful masses, active/passive participation?

Our priest would advise us to go but not partake in the sacraments. Weddings are joyous occasions and there is no need to provoke a possible life long bitterness from your friend who may resent you not showing up. His conversion will just have to wait

THIS ^^^

All of my family are novus ordo or Indult.   So, we have had this question come up many times.

We will attend their baptisms and weddings and consider ourselves as witnessing that everything was done validly in case they should ever become traditional.  We sit and watch (sometimes stand) but do not participate. 

We will attend their baptisms and weddings and consider ourselves as witnessing that everything was done validly in case they should ever become traditional.  We sit and watch (sometimes stand) but do not participate.

Yes, though I think that we are dealing with a Tridentine Mass here, not some non-Catholic Rite, unless you're dogmatic una cuм, I don't see why you couldn't "particpate" and must assist only passively.

Pax, Catholic theologians have long said that it's permissible to passively assist even at non-Catholic ceremonies, weddings and funerals in particular.

Pax, Catholic theologians have long said that it's permissible to passively assist even at non-Catholic ceremonies, weddings and funerals in particular.

No, that's incorrect.  

1.  Such an "allowance" was made for persons such as politicians in order to attend such for political, govt reasons.
2.  Such ceremonies are not simulating catholic rites and the scandal is minimal, as no one would think that a catholic attending an Anglican funeral for a diplomat was worshipping or converting to the Anglican religion.
3.  Such an "allowance" does not apply to catholic rites, which the indult is.
4.  Canon Law's prohibition of attending doubtful sacraments/masses overrules the above allowance.

How can you argue that the indult is both non-catholic and at the same time catholic?  

No, that's incorrect. 

1.  Such an "allowance" was made for persons such as politicians in order to attend such for political, govt reasons.
2.  Such ceremonies are not simulating catholic rites and the scandal is minimal, as no one would think that a catholic attending an Anglican funeral for a diplomat was worshipping or converting to the Anglican religion.
3.  Such an "allowance" does not apply to catholic rites, which the indult is.
4.  Canon Law's prohibition of attending doubtful sacraments/masses overrules the above allowance.

How can you argue that the indult is both non-catholic and at the same time catholic? 

You've got a good head on your shoulders fella. Too bad others let human respect get the better of them.

This is a wedding, not just a Mass.

Is it just you or do you have a family as well that will be going to the wedding? I ask because the advice below is just for an individual.

I say witness the two getting married.  Step out of the chapel for the wedding mass.  Enjoy the reception.  Don't make a big deal about it.

At my church the vows are done before the Mass, is that how it is done in the Indult?

It's not human respect, dufuses.  Catholic moral theologians hold that one may attend, though not actively assist in non-Catholic worship.  There's no threat that anyone will construe your attendance at a wedding or funeral at a non-Catholic ceremony as tantatmount to your intention to convert to their religion.  You go, in the case of a funeral, to offer your condolences and pay your respects to the family of the departed, and, in the case of a wedding, to witness and congratulate the couple and family for entering into a contract (and, in the case of being at a Tridentine Mass where the couple are both baptized Catholics, also a Saacrament).

Obviously you cannot go to a wedding that's invalid ... since your attendance there would be construed as your acceptance of something having happened there that isn't just a mutual agreement to cohabitate.

And, in the case of a Tridentine Mass, the worship there is NOT "non-Catholic" simpliciter.  It is in fact a Catholic Rite offered by those who themselves profess to be Catholics.  If you can't tell the difference between that and going to, say, an Orthodox Liturgy, where they openly profess to be separated from the Church and in schism, then you have some issues with your thinking.  Even the dogmatics SVs, like Bishop Sanborn, hold there there's a difference there, and that is why they accept people coming over from the Conciliar Church without the abjuration of error that would usually be required of someone coming over from a formally declared schismatic body.

Now, as to whether you can actively participate, it depends on your position about the una cuм issue, i.e. whether you commit grave sin just for that reason (I don't believe that), and your position about the validity of the Mass (depending on the priest offering it).  I hold that the Mass is of doubtful validity, but my opinion and $5 might get me a small coffee at Starbucks.  Since the Church has not issued a verdict on the matter, no poster can impose his opinion on your conscience.

So the guy calling even a passive attendance at a Tridentine Mass (a Catholic Rite) offered by people who profess to be Catholic (and most likely have the faith, given their attachment to the Tridentine Mass) a "compromise", even though you're not actively doing anything ... whereas for him it's perfectly licit to vote for (enable the election of) a Pro Abortion Jew-Puppet Genocidal Maniac.