No; there is no dispensation from form for the marriage of two Catholics.
Take place on the beach somewhere (only 1 person is Catholic)?
Is the other person baptized?
Take place on the beach somewhere (niether party is Catholic)?
Not a problem at all. Presuming that the couple freely consents to the marriage, and have no impediments to marriage, they can have a valid marriage at a beach ceremony. (And, if both are Christian, then their marriage is a sacrament, too!)
- If my best friend is getting married outside the Church in such a wedding for example, can I still attend such a wedding as a guest if invited? According to the Church, would this really be a marriage? Or does the Church view these people as sinners and not really married eventhough they are legally and people in the “outside” world refer to them as husband and wife?
This is a tough one, and I’m sure you’ll receive a variety of opinions. Let’s start with the objective questions:
“is this really a marriage?” You don’t mention whether your friend is Catholic (and therefore, is bound by the form of marriage). “It might be valid” is the best we can do, without additional details.
“Does the Church view these people as sinners?” The Church views all of us as sinners.
(But, to your question, if a couple is not validly married, and is living together, then yes, they’re in an objective state of sin.)
Now, the opinion question: should you attend? If there are Catholics involved, then there are two questions: are you showing your assent to the (invalid) ceremony, or are you merely keeping the door open, as it were, to opportunities in the future to convince your friend to marry validly (in a way that you would be unable to do so, if you blew off his wedding)? Secondly, is there the possibility of the sin of ‘scandal’ (that is, might others look at your presence as your way of saying that the Church saw this ceremony as valid)? A prudential judgment on your part, with respect to these questions, will lead you toward an answer…