What's the best way to interact with someone who has DID?

My advice in a situation like that is the same as it would be with anybody, which is the best we can ever do is to be with another person in the place that they are at. If somebody is experiencing drastic shifts in personality state, or moods, you're not going to change that no matter how much you tell them to stop doing it. You can always share your observations, but you're not going to change it, and you're not going to stop it. So it's a matter of, how do you sit with it and be supportive, while also maintaining your own boundaries, and not feeling like you need to get caught up in whatever is going on for another person. And oftentimes, the best way to be supportive for somebody is to ask them individually, how they need to be supported.