In Canada, and in business, pens, pencils, rulers, staplers, and other non-paper products are called office supplies (in schools, they are called school supplies), whereas paper products are stationery. Now, that's not to say that pens, pencils, and so on can't be considered stationery. They are, as you know, listed as such in modern English dictionaries:
Two online dictionaries (Click here) define stationery as follows:
1. writing paper.
2. writing materials, as pens, pencils, paper, and envelopes.
1. Writing paper and envelopes.
2. Writing materials and office supplies.
Moreover, the stores where North Americans buy paper and non-paper products are called either stationery stores or office supply stores. So, those non-paper products are indeed stationery, but... The term stationery store these days tends to refer to mostly paper, cards, and envelopes, but they do sell non-paper products too.
In short, who's right here, the dictionaries or the native speakers? Well, if you take a closer look at the dictionary entries provided above, you'll notice that the first entry refers to paper products; the second entry refers to non-paper products. In other words, the more prominent meaning is listed first. Which is why your American colleague feels that stationery applies more to paper products than it does to pens, pencils, etc.
Hope that helps.
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