If the invitation was for Surianne and Guest (unnamed), then I think you are fine to bring someone else if you want to, but not obliged to. If you choose not to, I'd call the bride as soon as you make that decision to tell her that circumstances have changed and you won't be bringing a guest. You can go into the reason, but aren't obliged to. I don't think you need to offer to pay for the meal since the bride isn't incurring any extra cost by the person cancelling.
If the invitation was for Surianne and a named guest, then I wouldn't assume that you could bring someone else if the named guest for any reason (including a break up) couldn't go. It would, to me, be only slightly different than if a couple got divorced, I wouldn't assume that if one half of the couple backed out, the other could automatically bring someone else. In this case, I'd call, and inform the bride of the change in plans.