I have served as an elder for a few years and all I can tell you is that there is no definitive answer to your question because it really depends on the mentality and culture prevailing in the body of elders of that specific congregation, more exactly those 3 elders who will form the specific committee for your wife.
The marked has no reason to exist as a consequence of a sin of immorality (porneia) but it is a gray area which is resorted to in borderline cases where the problem is disordered conduct. In cases of serious sins there is always the intervention of a judicial committee.
I would add that my personal opinion is that you two stick together regardless of what that committee decides. If the decision should be for disfellowshipping be prepared and expect much tears and emotional distress from your wife. Be ready to be understanding and never judge her, be loving and patient. Only this will help her overcome the pain.