This is interesting. Our average humidity is 80% and only that low due to seasonal wind reversals (esp. in the winter) where the humidity drops to 10% for a few days at a time. Here is our recent history, by way of example:
I have very few fungal problems with my succulents, though. I guess that can be attributed to excellent air flow (near-constant ocean breeze) and strong light, despite the regular nighttime fogs.
When in doubt, I recommend good air flow and strong light as potential preventative measures for fungal issues.
None of this may save the plant in the original post, but perhaps there's some food for thought.