Q.Is it better to clean up perennials or leave them for winter?
Is it better to let plants die down in fall and winter and remain as fertilizer for coming year (perennials) or should the plant stalks be cut down and removed? Argument between me and another gardener.

Your both right!
If the leaves are left to die back and remain in the garden they can provide some protection for winter, but they don't fertilize the garden.
If you cut back all the plant material in the fall, the garden work is lessoned for spring and these clippings can be composted.
Many gardeners do like to leave many plants in the garden for winter interest and birds feed on many of the seeds of spent plants.
Beneficial insects and caterpillars find homes in the plants that are left for the winter.
I do some of each, cleaning up some plants and leaving many other for the winter season.
https://www.gardeningknowhow.com/garden-how-to/info/cleaning-up-garden.htm