I'd say its best to get a trainer to come see you both and your pup for the most appropriate advice... its hard to say why he would react this way to one of you but not the other without seeing the whole picture

Has your partner tried leaving the pup at short intervals then build up the time, so a few mins at first, then longer and longer so the dog is happy and confident that they are coming home. I agree that SA has little to do with dominance, i think people use the dominance theory too flippantly for too many things without a full understanding of individual situations.

Also, buster used to be a naughty pup when left but we made leaving very matter of fact! didnt make a big deal of leaving, just made him sit as we got ready to leave and left - no big drama or excitement for him all very boring altogether. Now he just sleeps when we are out!