Permanently removing mortar under old Cast Iron Tub in a 2nd floor apt - by perhaps an idiot?

I live in a house built around 1890 with 1 apt on first floor and 1 on 2nd floor. It was a solidly built home (mansion) in 1890s, but the additions and jerryrigging in the 40s and 50s were/are sub-par to add on 4 rooms below and 2 rooms plus outdoor long porch on 2nd floor.
Both apts have cast iron tubs, theirs above my dining room (an addition). From memory, their BathR is approx the width of 2 cast iron bathtubs plus a sink on the opposite wall, and length of the room is the bathtub plus toilet and tiny linen closet. There is an outside wall on 1 side (toilet side), and 'rear' (sink wall), before the additions were made to the rear of the house.
About 18 yrs ago, the upstairs tub began leaking, supposedly at the drain OR the overflow. That eventually made my ceiling fall through (from side wall to end of their tub). Their inflow water for toilet tank at side wall was also leaking into my ceiling. Plumbers said they could not get to the tub leak because of how beams/cross supports were set up (and landlord did not want the expense. So they covered the hole with rocklathe and left it). Their continues to drip into my ceiling from time to time-- sigh.
Today, the new tenant tells us he is chipping out the old mortar where the cast iron tub sits-- literally pounding on it with a hammer / chisel. Says it is not needed! I argued it IS needed to distribute the weight of tub+water+human -- he argues the joists (which are partly rotted from water leaks) are all that is needed. I read through this which seems to support my arguments: https://www.homeownershub.com/maintenance/floor-load-needed-to-support-cast-iron-tub-207333-.htm
So how do I get a "dumb hick" who thinks he knows it all to Stop, Look, and Listen before a heavy tub breaks through my non-ceiling-covered only with a piece of rocklathe sheeting?! By the way, the guy told a friend that his floor has FIFTY-by-TEN joists-- laughing (can someone tell who sells 50x10s? I do recall dad nailing together joists when load required it and adding cross supports , but none were 50'x10' --oh my!).
Want to thank my very demanding and exacting dad who built our house around us as we lived in it, and who made his daughters/me "help" him from the time we could crawl and walk. Learned lots of lessons for every aspect of home construction. I may not always know names of tools or products, because I was so young when "building" with him.... but I sure remember how he constructed and his attention to details. And his warnings, and explanations of why were were doing X this way and not the "easy way".
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On Tuesday, June 6, 2017 at 12:14:05 AM UTC-4, cageyc wrote:

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The tub is typically supported by wooden joists, not concrete. You don't s ay why the guy is chipping away. What the extent of the problem is, no one here can tell you. Could be just a continuing leak that has done little str uctural damage or the joists can be rotted and failing. Don't they have cod e officers where you live? A tub leaking into the downstairs apt, you could n't get a co for a new tennet here.
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call the local building inspector. the long term risk will get the property owner cited.....
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On 6/6/2017 12:14 AM, cageyc wrote:

I'd start by reminding him that you are the owner and he is a tenant and has no right to do it. He can be sued for damages.
In any case, it sounds like an inspection may be in order if structural damage is a concern.
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On Tuesday, June 6, 2017 at 9:00:45 AM UTC-4, Ed Pawlowski wrote:

The poster isn't the owner, he's just another tenant.
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On 6/6/2017 11:27 AM, trader_4 wrote:

Then he should call the owner.
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