If it was me Id show the wife the messages. Did your daughter keep them? If not its trickier as you telling her this without evidence could backfire whereby youre the bad guy if he comes up with a good story. Frankly I wouldnt worry a second about "outing" him because if hes doing this with your daughter hes going to do it anyway, and if shes your best friend she deserves your honesty and support as much as your daughter does. She certainly doesnt deserve a sh"t of a husband like he sounds!
Either that or confront him but certainly dont ignore it. If it escalates it could go in all kinds of horrible directions, and if youre afraid or intimidated to confront him personally, contact the police and have them deal with it - she might be 18 but its still harrassment if she isnt reciprocating.
And by all accounts it sounds like youve done a fantastic job raising your daughter that shes handling this harrassment so well