Does your friend have type 1, type 2, LADA, ... or do they not have diabetes?
With type 1, I find diet drinks do not affect my BG.
However, I very rarely drink them - I would rather have naturally sugar free drinks (or take insulin with something like fruit juice.)
I usually avoid caffeinated drinks because they keep me awake too long ... unless I am very sleepy and need to stay awake.
Herbal tea is my drink of choice (many different types of herbs depending on how I feel). If I want something cool, I make them into an iced tea version by brewing a small strong version (teabag, 1 - 2 inches boiling water and leave to steep) then top the mug up with ice and cold water.
Edited to add: Ooops - just noticed this was posted under the Type 2 sub forum.
I will leave my post in case anyone is interested.