Must disagree with your 'disagree'.
Teach the new knitter to cast on with a larger needle, and then knit the first row with the smaller, gauge needle (bamboo). In my experience, a tight cast-on row is a 'lead-in' or 'set-up' for tight knitting. This can be avoided by the teacher's recommendation. Tight cast-on's are the first thing I watch for, and immediately correct. I teach & emphasize that their needle should always be able to slide back and forth through the stitches on them. Tight knitting can be an impossible strain, and very discouraging.
So, I still maintain that given the two choices she asked about, metal or bamboo, bamboo is best. When I'm teaching a new knitter, I'm called on to make a recommendation to her. I can't tell her 'it depends'. Bamboo will give ultimately give her more control, and fewer dropped stitches. If she has the budget, I would recommend Harmony's. If not, JoAnn's has inexpensive bamboo for a learner.
I think the 'right answer' is a decision.