She has put in rows of purls and knits arranged in triangles, see how at the top row there are lots of k stitches? That is the top of an upside-down triangle. On the bottom row, there is only one k: that is the bottom point of the triangle.
I have just removed the purls here - see the triangle? (Ignore the horizontal lines, I had to add them to get the ks in the right place)
This should make triangles, but after the fabric is felted, I think the triangles will not stand out, felting makes the stitch definition disappear. I think the triangles will be invisible or you will have to look very closely to see a difference.
I think another colour would be better for a felted item, make sure you knit a little practice square with triangles in these same two colours if you do try it, and felt the square. Some wools and different colours will felt differently and give you a bad effect, so check first.