Well there is no need for a temper tantrum. I agree with you in that I built my thief in a similar fashion. However using different traits and play styles across your different characters is what makes the game fun. My warrior is 100% glass cannon with over 100 critical damage. Yesterday I hit someone in WvW for 18k (they were not wearing armor and were upleveled, but whatever , I still hit level 80 warriors for 10k). That's fun... for that character. I don't want every single character to be the same, so I'm crafting a condition damage mesmer utilizing confusion. Entirely different play style, and that's ok.
If she wants a condition based thief, why not try to help her create a good one rather than raging at her for not being the most effective possible thief that is useful in level 40 FoM. She clearly is aware of what she's doing since she doesn't want to use a SB.