The old assessment was likely based on a much lower assessed value. 

Think of the worst thing that could happen, assess the risk of that event happening and reason backwards from there to a number (premium) you can tolerate. 

An example is a property that was last assessed for  96K, the previous buyer bought the property for 168k and has it listed for 260k(i am thinking wholesale).  

To video, tour or assess.

Condition and "fanciness": With the above searches, how nice is my home compared to the competition in the above searches.If my home is mid-way with fanciness, I would expect my proposed rent to be just a bit under the mean of the homes of a similar size and of a similar room count.The following additional factors are my speculation because I don’t have any rentals with top end interiors to compare with.If there are factors outside the above criteria, try to assess how much value they are actually worth to the target tenants.

Assessments are inevitable, but if the condominium has good reserves and is not currently considering any major capital improvement projects then it really could be a solid investment.

You will want to check w/various city agencies to see what kind of code violations may have been assessed and may continue to be assessed during the redemption period. 

This will help give him the information he needs to propel assess.

I am looking for a free rental valuation site
Not a rentometer

So I ran the numbers using your assumptions for vacancy, maintenance and capex, and management; last year's property tax bill, escalated a bit to account for an increased assessment; my current insurance premium; and the Zillow estimates for my home value and monthly rent (not ideal but hopefully good enough for a quick and dirty estimate).