I have never heard of giving vitamin B-12 shots for weight loss. From what I've been told, usually, if you have a vitamin B-12 deficiency you experience weight loss. Therefore, logically it would seem that adding vitamin B-12 would reverse that, but I'm no doctor. There are cases (such as mine) where there is a severe deficiency, but no weight loss.
As a side note, I would be wary of any Dr.'s that are promoting themselves specifically as "diet doctors", but that's just me. IMHO it seems that they are out to make $ more than anything.