What a sweet thing you’re doing for your friend. Office sticker paper is generally fairly innocuous - usually just treated paper.
The bigger problems you’ll get trying to use it is that you’ll need to print the color on it first (unless white or neon is what you’re going for), the adhesive on them isn’t designed for walls so won’t stick well and you’re going to need to piece it all together as the sheets come in only letter or tabloid size.
Cut vinyl lettering is kiss cut - meaning they cut just through the top layer leaving the backing so they can weed (remove unwanted vinyl around letters after cutting) put a temporary sticky sheet on top to transfer the design in one piece and avoid lining up the pieces individually (vinyl transfer paper, often used as masking for laser projects) This masking is likely to pull off the top layer of ink and paper from your office sticker sheets.
Definitely do NOT try actual sign vinyl in your GF, not even a little tiny word, you’ll destroy the mechanisms.
TLDR: It’s probably safe but really not a good choice for the project you describe.