Vincent,

The Catholic would need to have the marriage made valid if it isn't.

I trust, based on this line of questioning, that the Catholic Church did not give a dispensation for this wedding and that it is therefore invalid. The Catholic is therefore not able to receive Holy Communion.

However, if neither were previously married, it would be a simple matter of meeting with the priest and having a small ceremony to validate the wedding.

If the Lutheran in the relationship can't do this, then the Catholic should meet with his/her priest and ask to help have the Bishop sanate the marriage. (If the priest doesn't know what you are talking about — call the diocese).

Fr. Jonathan