I never knew that Oliver Cowdry was excommunicated because he had accused Joseph of having an affair with Fanny. I was never actually really clear on why he was excommunicated...only that he was, and then later came back to the Church.
Oliver was excommunicated for a number of things and this was only one of a list of about six or seven accusations. Interestingly, in Quinn's first book on the Mormon Hierarchy he notes the Cowdry was actually upset that Joseph took Fanny as a plural wife but at one point Joseph got after Oliver for an affair. That is the only place I have read that Oliver had a dalliance as well.
But one of the main reasons Cowdry was ex'd was for selling his land in Jackson County when the saints, at least at that point, were told not to sale. in my opinion, he really eas muscled out by Rigdon who wanted both Oliver and David Whitmer out of the way.