**WWJD? ** Do you really think that sharing scripture and Canon law is going to make a difference to a person who doesn’t know their faith? Obviously your presence is important to this person or you wouldn’t be asked to be involved and invited to witness.
And its in the witnessing that you must be most Christ-like. Gently, lovingly quote scripture/Canon law but in the end you must make sure that your absence won’t have a profound or long term affects on the relationship.
I have a sister who is going through this very thing. The love of her life is not Catholic and its affecting her. In our case, pulling away from her would be devastating. So there is great care, love and prayer when we approach her about things, etc.
Again, what would Jesus do? He came for the sinners. There was always great love behind the correction and instruction. And while it will not be a Catholic marriage, there is always hope for a reconciliation and Catholic blessing.
You and your situation are in my prayers.