I think Harley said it nicely. You can be honest with her without being hurtful or sounding like you're accusing her of anything. Just make sure you use feeling statements ("I don't feel comfortable with this" or "I'm having trouble dealing with this" instead of "You shouldn't be celebrating a year when you relapsed with me").
I totally understand why you feel the way you do. I also don't think it needs to cause any problems. You've already stated that it's her program and she can work it how she wants... you're just choosing not to celebrate an "anniversary" with her. It was her choice not to admit the relapse; it is totally your choice not to celebrate with her. As long as you're not badmouthing her to other people.
Good luck.