Friends and Open Relationships

Carly
Background: Two friends have been in a successful open relationship for 5+ years (2+ years married). Enter the third friend (me). Mrs and I have kind of lost touch because of work and what not. The Mr and I are friends and talk quite frequently.  Mr and Mrs have discussed inviting yours truly to a threesome which isn't a big deal to me.
Issue: Mrs is living in another state and Mr wants to exercise his open relationship rights...with me. If I agree, does that make me a bad friend to the Mrs? Even if she knows and consents? Can anyone help clarify how this whole open relationship thing works with a married couple and if they are your friends?