Unless he used low-loss RG8-style cable, such as Times Microwave LMR-400 or Belden 9913, and properly installed quality connectors, your reception will be compromised.
You keep referring to P25 traffic. Whether it's P25 or not, you still need an effective antenna
system at the frequencies involved. The antenna system includes the antenna and the feedline that runs to your radio. The Indiana SAFE-T system frequencies are mostly in the 800 MHz band, though some are in the 700 MHz band. So, you need an effective antenna
system at those frequencies.
While discones are the jack-of-all trades in the radio world, they are the master of none. Your antenna's receive capability may be OK at VHF, but not-so-OK at 800 MHz.
Combine a poorly-performing antenna with poorly-performing feedline and you have a real problem on your hands.
But, let's back up a minute...You also mentioned using a Uniden Home Patrol 2. Your scanner should be perfectly capable of hearing and decoding SAFE-T. Does your scanner have the current firmware? Have you updated the database? If you are still running the same scanner you took out of the box a few years ago, you may easily be missing things because the systems you are trying to monitor have moved on and changed things.
But, let's back up again...You mentioned that your HP2 receives just fine with a different antenna. That certainly points a finger at your antenna system.
If I were you, I'd take a critical look at your antenna and feedline before I considered buying a new scanner.