First, this hasn't actually happened yet, has it? You said "ends up with 140K cash in hand". If the taxpayer actually has it "in hand", that pretty much voids the 1031 exchange.
You are correct, ProSeries does not do multiple exchanges, so you need to manually do it.
The 8824 is to defer the gain and put it into the new property. If he is deferring the entire gain (which seems to be the case), it doesn't matter if he is investing an additional $50,000 or not. The entire gain is deferred into the new property.
So Line 16 (from Line A of the worksheet) shows the $190,000 FMV of the new property, which partially includes that $50,000. If he did not invest the extra $50,000, Line 16 (from Line A of the worksheet) would only show $140,000 for a lower priced new rental property.
Does that help at all? These things are confusing, so if you need further help, let us know. Maybe post screen shots of your 8824 (and the worksheet) if you aren't sure if things are showing up correctly.