1). There is no cure for separation anxiety. You can only manage it. It’s in her best interest to make sure she doesn’t get it. If at some point he has to go back to work, she won’t be able to cope. You’ll end up rehoming her and she won’t be easy to find a home for. Especially considering she will be one of many pups suffering the same fate. His work might have said they are permanently working from home but this is a dog that is going to live for 10-15 years. We’ve just entered a recession - what if he gets made redundant? Or his job changes and he wants to leave? 10-15 years is a long time to say your job situation will not change at all. Even if his job doesn’t change - does he honestly not want to go for a meal, a wedding, a funeral for the next 10 years? What happens if someone has to go to hospital? What will you do then?
2). You start small. If you’re both working from home, put the dog bed in another room and send her to her bed. Keep doing it when she gets off her bed. Small amounts of time at first. Get ready to go out, grab your keys, put your shoes on and then sit down. If they have a crate, put her in there at other times for example when you’re hoovering. So she gets used to going to bed/crate at times other than being left. Get into a good routine when you leave, our dog has to get on her bed and she gets a biscuit. She is told to leave the biscuit until we are at the door and as we reach the door we allow her to have the biscuit. So us leaving is timed with having a biscuit. Try not to be anxious on leaving, she’ll pick up on your anxiety. Go out for a 10 minute walk together, then a 20 minute walk. Build up the time but throw in a few 10 minute sessions here and there. This way she knows sometimes you come straight back, sometimes it’s a bit longer.
You might want to install a camera or something to listen to her. We used a sleep recorder app on our tablets to listen to if she was getting distressed, then you know if you have to slow it down. Observe how she’s getting on and change your pace accordingly.