Multiple Personalities Support Group
Dissociative identity disorder is a diagnosis described as the existence in an individual of two or more distinct identities or personalities, each with its own pattern of perceiving and interacting with the environment. At least two of these personalities are considered to routinely take control of the individual's behavior, and there is also some associated memory loss,...
http://www.pods-online.org.uk/partnersandsupporters.html
I think the best thing to do is to ask your friend how you can best offer support and caring.
Sometimes people get really fascinated with multiples and ask them if they can "switch" or ask, "am I talking with someone different now?" And for a lot of us that feels pretty disrespectful.
Just letting your friend know you're there can be plenty.