So I was in the middle of making my SMBC and got pulled away to attend do something and didn't get the chance to put my butter in. Now the meringue is super dry. It's been out overnight. Should I go ahead and just add the butter or is that a bad idea? Any way to salvage this situation? Thanks in advance!

I think that you should start all over.  If you use the meringue that you have, it might affect the texture.  Good luck

take a bit of it and add the right amount of butter and see how that does before you invest your entire load of butter into a maybe --

if you would post about your results that would be nice -- and best to you