i think it would have been far worse if your friend had made the judgement that you would be unable to attend and so had left her off the invitation. When I invite people to a celebration which will cost them money because they will need to stay in a hotel or B&B I encourage them to make it part of a holiday in the district and an opportunity to see relatives and friends. All I can offer is space for camping.
I think sweeties suggested response is a good one
“Thanks for the invite, unfortunately I can’t make it but I’ll be thinking a of you, have a brilliant time”
But you might preface it with 'Thank you for including me in your inviation to your party, but unfortuanely etc.
Then you can sent a present/flowers to your friend for the day.
You should be pleased that your friend thought to include you.
All the best.
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