Just wondering if the OP has decided on a course of action?
Yes, I've decided what to do.
No, I haven't taken any action yet.
With the rain and the drastically falling temperatures that tarp is pretty much frozen solid. (I did physically check on that . . . it's stiff as a board.) So the timing is just bad. There's really not much she can do about it now.
I wanted to talk to her about it yesterday (after WillyNilly's post) but her house is dark with no car in the driveway, both yesterday and today.
We're suppose to have a warming trend next week, so maybe the tarp will thaw out. Next time I see her car in the driveway I will approach her and mention the wood-rot damage to the fence and ask her to remove the tarp.
I'll keep you posted.