Tropicals & Tender Perennials: What should I do with this begonia?, 1 by fancyvan

fancyvan wrote:
I am overwintering some begonias for a friend.

The one in the pic had powdery mildew and I gave it a few sprays of baking powder mix(baking soda? dont remember which now!) and that seemed to control it. However many of the leaves are drying out starting at the edges. Also there is a very long stem ( somewhat like I have seen on African Violets ) and I am wondering what is best to do with this - especially since it is not mine! Just clip off the leaves that are drying or cut back the stem and let new growth come up and root the stem I cut off? Not sure how but seems to me that can easily be done?