He should have told her from the very first. It's just plain rude.
If i were that girl, I would feel extremely hurt. It happened to me; I agreed to a casual relationship but I didn't find out for a while that the reason we were being casual and not official was because he had also started seeing another girl and i had originally been under the impression that i was currently the only one.
It hurt a LOT. He is going to hurt this girl and she will feel betrayed if he doesn't let on that "seeing other people" means "seeing one specific primary person for the past year, oh yea btw she knows all about you."
it just seems inconsiderate. She should have a better idea of your relationship dynamic besides "seeing other people" which could totally mean just random hookups or other casual interests, not a long-term primary relationship.