The unfamiliar smell made him hiss at her bag and then since the other 2 cats were there, he displaced his aggression (fear, defensive mechanisms) on the other cats. This is a very normal occurance and the best thing you can do about it is DON'T REACT. Don't make a big deal about it. Let him get over it on his own. Don't pick him up to reassure him when something like that happens. He could also displace his aggression toward you.
In the future, it might be best if your friend's things were left where he can't get to them. Of course, remember that Dexter was only following his instincts. He sensed a threat and reacted to it.
Thank you for the adorable picture!!