From a legal standpoint, and based on the OP's description of what happened...
he didn't technically "poach" the deer. When he shot the deer, he had a legal tag in his pocket and the season was open.
What he did was:
1. Fail to tag it immediately after the kill.
2. Bought and used a tag under fraudulent circumstances, since it was his wife's tag, and he himself killed the deer, and had killed it before his wife bought the tag.
So he's likely charged with fraudulent use of a tag (or whatever that state calls it). I don't know if it's a felony or not, but it may fall along the lines of "perjury" or some such where his wife signed the tag, and he'd already filled it.
And if he's not careful, he may bring his wife into it with some sort of conspiracy charge.
He needs a good lawyer.
Edited to add: Sorry, should have read the entire thread before posing. Looks like someone else already posted the same thoughts I had.