In the 125cc commuter bike segment, if you want a straightforward, simple machine, your best bet would be the Honda Shine. This is a machine that has proven its dependability over the many years. It has been on sale and will be reasonably comfortable for two-up riding, as well as return excellent fuel economy. You can also take a look at its more stylish sibling, the SP125. It is mechanically very similar but gets a few more features.
While you have said you want a simple, no-nonsense machine, the funky-looking TVS Raider is an option we would also strongly urge you to take a look at. It may look flashy, but as a commuter, it works extremely well and will serve you well.
The lower versions come with a single-piece seat, which will be slightly better for two-up riding. If you want the highest level of safety, then the top models come with single-channel ABS, which is a worthwhile feature to have.
Take your father for a test ride of these bikes and see which one suits him best before making a decision.