According to employment law , if you've worked for less than a month you're not required to give any notice.(It also works the other way-company doesn't have to give you notice.)
After the 1st month the minimum notice is 1 week, unless you've signed a contract that differs.
So, your friend can leave today with no worries.
Having been an employer myself, I'd suggest that they go to their manager or HR and just say that it's not working out for them. Then at least the employer knows and can replace them. I once had somebody just not turn up and I spent a lot of time trying to contact them and was quite worried in case something had happened to them.
And, they can walk away on good terms with a clear conscience.
Hope that helps
PS. I'd leave it off the CV too