I think if both are codependent, then they are not getting their needs met and sooner or later one of the codependents will find someone outside the marriage to 'fix', not as in an affair, but perhaps a sibling, child, parents, friend, etc... They will find someone who meets their need of being overtly messed up so they can play hero, once they realize they aren't going to get their hero fix from their spouse. Then the spouse can get his/her codependency needs met by trying to rescue the spouse from his/her codependency.
It would work like this: two codependents, each feeling increasingly dissatisfied and unfulfilled because all their attempts at getting their needs met by codepending on his/her partner are blocked by the partner who is competing for the same thing.
Then a baby is born, or someone's sister ends up divorced and homeless, or elderly mom breaks her hip, and VOILA, it's a miracle, now one spouse has a means of acting out codependently: immersing herself in parenthood; rushing to sister's side; becoming consumed by mom's medical and emotional needs.
The other partner is left out, sees the inbalance in his/her spouse's life, the consumption of time and resources, effort and emotion given by the partner to the outsider, and tries to rescue the partner from the problems of the third party.
And suddenly you are in codependents double hell. "But it's my mother, what am I supposed to do? Abandon her in her time of need". (I'm a noble saint! You are a selfish beast.) vs "You need to establish boundaries, delegate, disengage..." (I'm reasonable and only fighting for our marriage, I'm a noble saint!)
Co-dependents WILL find someone to meet their needs, just like an addict WILL find his/her substance of choice, and another codependent won't measure up.
Codependency is an dysfunctional strategy to manage life; any relationship in which codependency plays will also be dysfunctional. Dysfunctional relationships are unhappy relationships.
Sounds to me like your friend is trying to justify not having to learn healthier non-codependent ways of living. Maybe she feels if she finds another codependent, she doesn't have to work on herself.