I'm looking at purchasing a duplex for house hacking, however the one I'm looking at has fire damage on the bottom unit.

To date there have been no issues with damage and it still looks as good one year later.

Invoice them for damages, legal fees and get the property for the next tenant.

But I am curious to see if they actually repair the massively damaged foundation, or try to hide it by “finishing” the basement.

Since I don't know the extent of what he might do in the remaining couple months, I'm concerned that he may damage the condo or building in some retaliatory way, or do something else troubling.  

If his cutting  endEd up damaging the tree, then he could be liable.

You'll want to look at indirect things such as "warnings to residents not to leave food scraps in hallways" or "needing to replacing damaged wiring" etc. 

If they are existing tenants with a good track record, and they're not doing anything that's not reversible or risking damaging good flooring, it can be considered. 

Hopefully there is not much other damage and you can deduct the water bill from their security deposit. 

Basically, if your property is damaged by a storm big enough to have a name, you're covered.