It is treated as investment property and covered by a sect 266 election. Since the returns for those years have never been filed, you might be in luck.
Reg 1-266-1(c) states that the election has to be made with an original tax return, there is no reference that the return must be timely filed. Pub 535 also state the election must be made with an original return or if an original return has been filed, the election can be made with an amended return filed within 6 months of the due date without regards to extensions.
So it appears that since your client's partnership has never filed any tax returns, those years are all open and original returns can be filed to make the election.
However, you need to consider the effect of TCJA for years beginning after 2017. An election for sect 266 only covers otherwise deductible expenses. That limits the election to $10,000 of property taxes for years after 2017; and also eliminates the election for carry charges subject to the suspension of misc itemized deductions.
Unless I am overlooking something, it looks like you can file a 1065 to make the 266 election for each open year and increase the basis (subject to TCJA for 2018 and 2019).
$100,000 increase in basis should be well worth your fee for some basic returns to make the election per the reg and IRS pub.
In situations where the election was omitted on original returns, PLRs have ruled favorably for taxpayers that were not advised of the 266 election, but that does not appear to be the case here.