So we're talking about a Serious optimization problem. I understand that the colleague's PC is not top of the line, but we're not talking about a AAA game either.
His first game had a major issue that affected performance with it getting progressively worse over time, and that same issue is present here - How he's making the game. Which means it's worse than just optimisation issues.
If you look back through the thread, you'll see instances of people appearing in locations that they "shouldn't" or multiple instance of a character where there should only be one. Each location is having every possible character for all events that happen in that location being loaded, and the ones that are not supposed to appear are (supposed to be) just hidden.
This means that some locations wind up becoming more and more filled with events and multiple instances of the same characters all being loaded in, resulting in an objectively high performance hit - It's ameliorated by hardware (SSD, RAM, GPU and to a lesser extent CPU) but more than it realistically should.
I suspect the clothing customisation is, or will, make things even worse since it adds another level of complexity to the proceedings, especially as it's hardly bug-free itself.
However, this game is more graphically demanding than The Twist was, and even that game had problems running with integrated graphics like ssenger67's Intel HD630 (which is 8 years old now). Then again, Intel's integrated graphics aren't really known for being decent for gaming as it is, but that's been a problem with any integrated graphics chip/drivers in general for a long, long time.