@Denita Ozenne
Hey there. So I am assuming that this is deal number one for you..... Might have missed that but I think that's right huh?
I would say that the answer to your question varies. If this is a really great deal then perhaps there is room enough to just let it go and close and deal with it yourself. However, if the numbers are on the tighter side then forcing the repair might be a good option to use and still close the deal.
I would assume that if you were able to get responses from a number of others on the site they would probably say things like I am going to list here.
1) Problems can seem small and simple at first but grow larger in a hurry - it happens.
2) Depending on the type of shower/tub installed in the above unit it may be more than a simple fix and get into much more money than you think - you won't know until you open the ceiling for sure.
3) After the plumbing repair you could be looking at mold potentially in the joist spacing area between units.
4) Depending on the length of time the issue has existed and been patched to get by you may also have damage to the wood - joists between units and or sub floor issues under the upper unit tub/shower.
5) If there is insulation between the two units, you may have soaked and degraded insulation that needs to be replaced as well - good news is that rock and insul are cheap to replace.
6) OR......... you may have someone open it up to fix it and its just a simple plumbing issue and is a quick and easy fix - totally possible too.
Just as an illustration, I went to replace a bath faucet for an accountant friend of mine yesterday. It's simple right? Pull old faucet off and reattach new one and turn the water back on right? NOPE - I had faucet off and new one on and then the drain piece broke, actually fell out bc of the pressure I had gently applied and was already broken anyway - just waiting to fall apart in my hands. The faucet was very old though didn't look the part. It was a major frustration given my expectation for a quick fix. What should have taken about 30 minutes or little over ended up with 2 runs up the street to the ACE store for additional parts, and 3 hours later - some of that due to the fact this is on main street and it took a while to get the water shut off figured out as it was all redone this summer by the city. Although it wasn't anything major it still shows how the faucet suddenly became the sink and drain assembly as well and combine that with the fact that the sink cabinet was so small I could hardly squeeze into it to work on things as it was. It wasn't just as simple as it had started out to be. Same could be the case with this situation you have here.
I would say that you can ask for the issue to be fixed, prior to closing is better so you can reinspect. Better option would be for you at this point to get a concession in closing from seller for an amount given by a plumber as estimated cost of repair so that you can fix it yourself (have it fixed), so that you are in control of the work that is done and know it's done right to your satisfaction and isn't just another patch job to get the property off sellers hands and into yours for the least amount of effort and cost possible (this does happen often).
Simple question with not so simple answer, sorry about that but hope that helps cover the bases. Some of this will totally depend on the age of your unit, so keep that in mind too. 100 year old houses can propose more issues than more recent construction.