That is a tricky one. The company I work for employs thousands so rules need to be followed and there are ways to deal with disciplinarys. A company with 6 people and the owners wife seems dodgy as is. Something like that would go down as corruption in our business alone (The owners wife being given a seniority role in big business without a fair interview).
I guess you could always take it to court but then your wife would have to leave after that anyway.
I will have some of what he has been smoking please.
Adam, you seem to be comparing a big corporate to a small owner-run business. The owner can hire whoever the hell he wants, interview or not. It isn't "corruption". There is nothing at all "dodgy" about a company with such small numbers having the owners wife / husband / partner as part of the management team. As for court, this is a massive overreaction.
At the end of the day if it is such a small business then they can choose how long their written warnings last for. What the real issue is really, is did both parties do the same thing, what was that thing, and could they both be proven to the same extent. That would then explain if the fact they received a different sanction was justified or not.
Since the OP is not going to discuss that, it really makes the thread redundant.