Isn't this a non-issue? You are responsible for the transaction, and nothing further than that. Flagged coins are stupid, and I personally couldn't care less about it. The transaction would concern both parties holding up their end of the deal and nothing further, unless you are supposed to be transacting in "untainted" coins. You shouldn't be responsible for whatever happens outside of the deal, and furthermore you are not in trouble for using a mixer because you can prove that you are not involved in any illicit activities.
It is an issue the moment the person sends the coins that they received from you into an exchange. When they get asked what is the source of these coins, they will just point to the previous transactions, and now you are the next target on the line for whoever is investigating that. So it doesn't matter what you don't care about, and yes, we are innocent until proven guilty, but why would you want to expose yourself to that risk?. You should just make sure that any coins you send are not tainted while maintaining your privacy. The question is, how to even go about that.