If you were to use the boxer as a condom and actually ejaculate in it, there would be a big chance of her being pregnant. But there is a very low possibility that pre-*** can get her pregnant (you can do more research as to why is that). Sperm stays alive in wet surfaces (in vagina) so if you did not ejaculate in her, don't worry yo much. Hope I helped.
Yes you did that helped a lot thank you.
No problem. But remember "there's always a chance" so if late period occurs or something of that sort, just get her to test. Good luck