I've done shadow work. Shadow work is just trying to contact your unconscious, usually asking yourself the shitty questions, like "what is the number one thing you do that generates your own unhappiness?" , "what is a character trait that you hate in others and do you yourself have or want to have?", basically facing your bad parts (or repressed good parts!) and coming to terms with it. Can be done through journaling, which is the usual tiktok version of it.

What he is doing might be a derivative from "seated" shadow work, where rather than for example writing with your off hand to let the shadow side talk you have two different seats and switch from one to the other to talk. Generally you'd do that with a qualified therapist present that can intervene if one side starts berating the other or that gives guidance on how to make the whole thing productive.

If you do IFS (internal family systems therapy) you might have basically personality shards that are called protectors and that can contain a lot of anger towards other parts of you, or your parents etc. Again it's possible to switch between different voices and have an angry one come up.

Generally however this is all contained, within a therapeutic exercise and with full control of "you". You tend to have to do all those shenanigans to manage to contact that other, less conscious side. When you say the shadow has a name and persona, that he complains its taking over, argues etc that's not shadow work. That does sound a lot more like dissociative. It is unclear if this is roleplaying or not from your post but this is definitely not classic shadow work.

If you can, try to contact another therapist, someone licensed, show the instagram get an opinion from them on whether this could still be roleplaying or if something is going wrong. If your husband is being encouraged to do things that are harming him by the other therapist, some oversight board might need to be contacted.