Open OBS (always helpful, right?).
Select the "Scene" she wants to add it to, then under "Sources" click the "+" button to add a new source. Choose "Media Source" from the options and either create a new name (I would, it's easier) - just call it "BRB Video", click "Ok". In here, upload the video you gave her, and also make sure to tick the box for "Loop".
Click "OK" once you have the video uploaded and you've selected "loop".
And then this will show a new source for you, presumably with the name you gave it.
Click the "eye" icon to hide the video and then click the "eye" icon again to unhide it when she needs to use it. Make sure the "brb video" source is ABOVE the other sources, so it would be something like this in "Source":
- BRB Video
- Webcam
If you put the webcam source above the video source, then as you can imagine it will show unless you hide the webcam. Basically it works as layers. You can move the layers above or below one another by selecting the source, then click the "up" or "down" arrow next to the "add" source button.
Hope this helps
Edit:
Note, she may also want to mute her mic when she goes for BRB