For the second part of his retraining, you will need very high value treats - something he absolutely adores, such a bits of warm turkey hot dog or whatever he literally drools over. And B4 you do this training, be sure he's very hungry.
Then, find out what his 'hot zone' is - what distance he's uncomfortable w/MIL at and begins to tense up, alert/focus on her, show teeth, growl, etc. Once you learn that, have some training sessions where she comes into the room with a treat, and just B4 she gets to his 'hot zone', have her drop a high value treat on the floor and stand there until he leaves you all and comes and gets the treat. Once he turns to leave, she can, too. But he has to come to her to get his treat and leave first. B4 long, he will associate her coming into his 'hot zone' with getting his wonderful treats and together with his other training when he actually snaps, start to lose his need to feel territorial and should grow to totally accept her anywhere near him.
__________________  Jeanie and Tibbe One must do the best one can. You may get some marks for a very imperfect answer: you will certainly get none for leaving the question alone. C. S. Lewis |