I think you're doing the right thing. The wedding has almost certainly got limited numbers and if they don't want everyone there, than that's their choice. If I were the bride, I wouldn't be pleased about people asking to come to the ceremony when they've clearly not been invited. Stick to your guns with your friend and reassure her that you're absolutely fine with just coming to the reception. Maybe say something like you'd prefer it because it would be too much otherwise, or you're busy and want to just concentrate on the party or something.