People are saying this fits on it.
Joyetech eGo-T 18350 battery MOD tube - Joye eGo-T spare parts Heaven Gifts
So you could buy the control head for 75ish, this for 6 bucks and be good to go. Since there is a mah setting for battery, it may screw up the "puffs left" predicter.
I know some people on here ordered that tube to try, you could wait and see the results.
Honestly, I don't find it to be that big with an 18650, it looks bigger in pics and video. If I was you, just get her the kit and by the time she decides if she wants a smaller battery or not, other people will have vetted the small tube.
Also, someone will inevitably tell you to get her a vamo and then list the reasons.