What do you think about testing a feature?

At work, in my time, there is another developer (we're 10 of us), not junior, not senior, just at middle. I'm junior, obviously.

We have developed a new feature, implemented in a form named "MyFeature".

This features is accessible by other forms in the software.

This is the point: today we had a discussion, I said that a tester should test a feature everywhere it is implemented. Instead, he said that the tester have to test the feature only in its specified form.

By my opinion, that's completely wrong. What do you think about? This confuses me...