Sounds like you are potentially inviting a leaky shower by penetrating the barrier if you do that. Even with an originally-built bench, there are mixed feelings about potential long-term issues due to standing water sitting on the bench. You'll probably get a lot of votes on here for a nice teak bench in the shower. That's what I'd do.

Reading here at GardenWeb, I know that tile and grout are not waterproof. Your guy is just not informed. Yes, we have bathrooms done "the old way" and sometimes the tile above the bathtub did stay for many years. But how many have rotten walls and floors behind them? Do yourself a favor and read up on how a shower should be built before you hire the next person, and talk about their methods and products before you hire. People are still out there, putting up materials worth thousands of dollars, not knowing what they are doing. After the leaks ruin other rooms in the house, all has to be torn out and redone. It happens all the time. Here is a current thread: http://ths.gardenweb.com/discussions/4654503/wall-underlayment-for-shower-best-practice?n=4

Contact Dulles Glass (https://www.dullesglassandmirror.com/). I had exactly the same situation as you (same size Kohler cast iron shower base in an alcove) and did not want holes drilled into the shower base. DG measured, built, and installed a sliding door (one side fixed, the other slides) without drilling into the shower base. Drilling through the tile was required to the hold the fixed wall and top rail. I paid $2025 after negotiation, yeah not cheap. For a DIY job, I had contacted Dreamline (sold at Home Depot) which told me that the following models don't require drilling into the shower base: Model: SHDR-1260728-01 Duet 56-60 in. W x 72 in. H Semi-Frameless Bypass Sliding Shower Model: SHDR-0960720-01 Infinity-Z 56-60 in. W x 72 in. H Semi-Frameless Sliding Shower Door Model: SHDR-1160726-01 Visions 56-60 in. W x 72 in. H Semi-Frameless Sliding Shower Door Here's a great video about installing a shower door. Note that he does NOT drill holes into the shower base. He's installing a Nadoli shower door. I don't know if that brand is available in the US (the presenter is in Canada).

My guess would be a tile installer, but I think that type of shower pan construction is a long gone skill. There are so many better products available now that I can't imagine someone using hot asphalt and roofing paper to build a shower pan. Bruce