I would say try not to worry about it. It seems like the lady you called out is the only one that is saying anything about it, probably because she is embarrassed about what happened. Some people have a very hard time admitting to relapsing so instead just lie about it. I don't think it's worth making it your problem though.
Does it really matter which one of them is being truthful?
Are you there to be the relapse police or to work on your own recovery?
Why would you want to be any less honest just because somebody else is lying? Why not just set a good example and make it a reason to not want to be like them?