That does change things. As someone already said, you may not know the whole story. But even if you it is what it appears it really isn't something that you can or should do anything about (bad as you feel). I would simply send my regrets, no excuse needed, and not attend.
I also agree with iheartbisou, I try not to mix my work and personal life.
We often judge people without knowing the whole story and there may be many things going on in this persons life that you are not aware of.