If you already know that someone will not be able to attend your wedding, for whatever reason that may be. You are probably wondering at the back of your mind whether to still send an invitation or not.
This is a great question and it is very thoughtful of you to think of them like this. However, since your friend has informed you prior to the wedding that they will not be able to make it, then it isn’t necessary to send them an invitation. This will save you energy, time and money because you know there will not be any positive results. Your friends will probably feel bad about missing your wedding due to prior engagements, so receiving an invite will just be a reminder of that.
It is proper etiquette to send those who received a save-the-date card a wedding invitation. Even after your guest mentioned they won’t be able to attend. It still may be important to send the invitation because there is a possibility that plans could change. Save-the-date cards are usually sent months in advance and that is enough possibility for your friend’s commitment to change in the lengthy time frame. If they still cannot join you on your day, they will still feel included. Perhaps speak to your potential guest as the wedding date gets closer and see if their engagements still hold; and if they don’t, then you can send them an invite.
By sending one less invite, you are also being environmentally conscious. But if you still feel bad over this, perhaps send them a card saying they will be missed at the wedding and wish them well on their prior engagement, whatever it may be.