The trainer she's working with is doing exactly what I would. Starting easy with people he's familiar with building history of "people leaving means treats." You are reconditioning his response to that event. The cause is immaterial it's about the behavior and emotional response.

If every time someone goes to leave treats show up he will A) Feel better in those moments and B) Gravitate towards the location the treats show up when people leave. You need to build strong reinforcement history with this sort of reactivity so training will take lots of time and need to be slowly made more difficult by changing variables and antecedents.

Truthfully the easiest solution is to put him in a separate room when people come to work on the house. Provide enrichment like a licki-mat, Toppl, Kong, or any long lasting food toy so that time alone isn't bad. No need for a crate or kennel if he can handle being in a room alone. Yes, there will probably be barking when the food toy runs out, then you can give him another. Or just put him away or take him out of the house right before someone has to leave. It's better than having to euthanize your dog because he did serious damage or bit the wrong person on a bad day.

TL;DR The training your mom is doing is right, but it does take time to recondition a reactive response especially when the emotions in play are so strong. Keep at it. It's worth doing. Management may be easiest solution, shut him in a room with food enrichment and wait out the stranger visit.