I'm an invited speaker. Can I send a coauthor in my place?
One question is whether your coauthor will do as good a job of speaking or otherwise contributing to the conference.  If you are well known as an excellent speaker and your coauthor is not, or if you are a much better known researcher, then the organizers may be unhappy with the substitution.  Another potential issue may be if your name has been used to advertise the conference, on posters or the web.  However, if you and your coauthor are more or less on an equal footing, then I doubt the organizers will object.

I'd recommend apologizing for being unable to make it due to reasons beyond your control (without going into detail), and offering your coauthor as a potential substitute, while still giving them the chance to decline and just cancel your talk or replace you with someone else entirely.  I'd make it clear that you are suggesting the coauthor as a possibility without having asked the coauthor yet, so that they don't feel trapped by not wanting to insult a substitute you have already lined up.