Powdery mildew spores cannot survive in soil or potting compost, but can be found on dried plant debris. If your friend--bless him or her!--can clean up all of the debris from the pot and surroundings, then that should stop that source of infection. Note that it is a good idea to completely change out all potting compost whenever you plant new plants, to help prevent the spread of soil-bourne diseases, such as phytopthora or pythium.
Remember that even with good sanitation, it doesn't keep spores from blowing over from your neighbors property, or a kilometer down the street! I would check and see if you can improve the light levels and air circulation in the area, or plant them somewhere else entirely.

Thank you so much Tugbrethil. I am not so afraid of tackling it now. Your help has been much appreciated.

Still, lots of us who can't justify the expense of new compost every year just renew the top part, or even simply add fertilser and make it do another season. I've never had any problems with diseases carried over but maybe we've just been lucky.

Works for some plants, and not for others, Steragram. Here in the desert, we have a phytopthora fungus that eats the roots of both Madagascar Periwinkle (Catharanthus roseus) and Petunias. Whenever we plant either in pots, we have to completely remove the soil, scrub the pots out, and let them sit in the sun for a few days before planting more of the same. Pansies, Lobelia, and Begonias can be similarly affected by Pythium blight, though that is less common here.

Thank you both. I am going to be as careful as I can be with having the greenhouse cleaned, the boxes the flowerpots are stored in, etc. Think I will get the first inch or two of soil taken off all the pots in case leaves have shrivelled up in there. You have given me hope now that I may be able to get rid if it before next Spring. Thank you.