Yes, this is most certainly the right angle, but very possibly the wrong photographer! I like the way the background is skewed, but the set square is exactly 90 degrees with the image edge. Quite crafty.
The strength of this photo comes from the girl and the object she is holding, not the fact that that object contains a couple of right angles. (The image would be just as strong if she was holding, say, a french curve.) The objects on the right side of the photo compete with the subject and should be cropped out. The glare on the girl's chest is a bit distracting, but I like the way the lighting is arranged to put the shadow just the right place.
Get a shirt on this girl. Who do you think you are--dr. JOnes? Just kidding. Cute gal. The right angle of the triangle is not as clearly apparent as it could be, but what a great idea to put in the random background of leaves and child.