Helping participants in usability tests

I've just completed a round of lab-based, co-present usability tests as part of a usability audit for a web-based map application. I have ten tasks for the participants that require them to perform typical map activities (zoom in on this area; find this location, measure the distance between x and y, etc).

The first task is to zoom in (there is a little magnifying glass in the toolbar that lets the user do this) on the map in a location of their choosing.

But for one of the participants, they simply couldn't find the zoom tool. This is good information because we can take actionable steps to make it more visible. However, a lot of the subsequent tasks required the use of this zoom tool; without knowing how to zoom the test couldn't be completed. So I pointed out to the participant where the zoom tool was. I realize this is a big no-no in usability testing but I felt it was necessary in order to complete the test.

My question is: Should I have intervened and helped the user or should I have ended the test? At what point do you abandon a usability test for the sake of keeping a more realistic context?