Oh but Arkade Prince, I am nothing BUT a disruptive influence, ask anyone.
The trick with this riddle are the assumptions people make. The third sentence reads, "he and his wife go to bed" which does not necessarily mean the farmer and the farmer's wife go to bed - in the morning, upon realizing the situation the farmer kills the farmer's wife upon seeing her and "him" in bed. That seems to be the simplest explanation, the one based on the least number of assumptions. More complex answers can be offered, but they demand greater pretexts and leaps of logic. I've seen this tale told with slow-acting poison and a myriad other off the wall suppositions. And, as Arkade Prince says the infidelity is not on the side of the wife, I think he's taking some liberties with context and premise.
Of course, if the wording IS correct, there is something very fishy going on as the action is either not chronological or takes place over three days. That is, he comes home at seven in the evening on Night 1 ("At 7pm the farmer comes home..."); she cooks for three hours ("...his wife is still cooking for dinner. He takes a nap,...") and they eat at ten in the evening on Night 1 ("...at 10pm his wife is done..."); they do not, however, go to bed until twelve noon of Morning 2 ("At 12pm he and his wife go to bed..."); he then kills her at seven in the morning of Morning 3 ("At 7am the man kills his wife..."). The third time is listed as "12pm" which is noon, instead of "12am" which would be midnight and the logical time for people to go to bed. Adds intrigue and complications.
Though, I think it would be interesting if "his wife is still cooking for dinner" means the old hen was in the stew-pot, and "his wife is done and he enjoys a nice hot meal" implies the farmer supped on his bonny lass. But, I'm a bit askance at times.
Dramthin Hartsboon
Simple Keeper of the Tainted Grove