This is something that Hugh would discourage, but, with the customers permission after informing him/her of the risks, I have had good luck by treating mildew stains with a 1 : 20 dilute solution of Clorox and water.
I use a cotton square dipped in the solution, squeeze out the mixture so that the pad is just barely damp, then gently rub it over the mildew. This should kill the spores. There may be some residual staining because of the mildew, but the Clorox will prevent it from getting any worse. And, sometimes, if you get lucky, the stain goes away completely.
Once this is done, go over the area with just a <u>damp</u> pad with water to get rid of the residual bleach.
Clearly, with any really valuable pieces, you would want have this done by a conservator, but for those cheapo prints that the customer doesn’t want to spend any money on, this has worked pretty well for me.