This question is one of the several reasons why the Catholic Church stipulates an extended period of time for marriage preparation, typically one year before the wedding day. The first step for the couple to take is not to shop for banquet halls or churches or exotic locations, but to sit down with their pastor so he can assess their needs.
Among many other things, like maturity of the couple, their ability to give consent, their freedom to marry, if there have been previous marriages, or there are any serious impediments, pastors are to assist the Christian party or parties to make an effort to improve their faith practices, as it is beneficial to the success of their marriage and future family.
If both parties are Catholic, and if one or both parties either refuse or does not show adequate effort to be more serious with their faith, pastors are to deny them the celebration of the nuptial mass on the wedding day, but instead offer to have the wedding liturgy without mass instead. It is necessary since it would be sacrilegious to knowingly offer Communion to a non-practising Catholic, or a sign of contradiction to have a wedding mass but only offer communion to one of the newlyweds.
As to the question regarding the efficacy of the sacrament of matrimony outside of mass, we have to realize that there is never any lacking of grace coming from God to the couple in every valid marriage. God is a God of overabundance. The grace He pours out on every marriage couple is always the same, infinite. That’s what God offers, on the wedding day and every day forward.
However, how much the couple actually receive would greatly depends on their individual disposition. If his or her heart is open to God, there is no limit. If his or her heart is closed, the whole universe full of grace would make no difference. So, pray for this couple. Pray that their hearts will be open, even a little, in his or her own way. It might not be full faith, or just a need. Just a little crack, we pray, and that might just be enough for God to come in and start something new. Amen.