Definitely not A. As soon as she mentions it during an interview, it's a factor. If she doesn't mention it, then her credentials get her hired or not hired.
Is this a complete hypothetical or does she have an interview scheduled? If there's no interview, then don't cancel something "just in case" just yet.
My bigger question is, what is your friend's financial stability? If she really needs the job, then postpone the trip. Pay the fees and reschedule. If she doesn't then less of an issue.
Most companies do not give you your "vacation time" at the start. You accrue that over time at certain increments. If she gets 3 weeks of vacation per year, then she could likely "bank" her days (meaning she doesn't take a single day off for any reason between now and the vacation) and work something out with her employer to "borrow against" the balance of her time.
The "OK" thing to do is to accept the offer and then tell her boss that she will need to take 3 weeks off. She should tell her employer immediately and then make whatever arrangements that he/she suggests. Being upfront and honest will always make a better impression than not.
On a much smaller scale, this happened to me. I started my current job last August and then had to take 5 days for an out-of-state wedding that I was in. I told my supervisor immediately upon accepting the offer. I let her know that I could cut-down on the days, but that I couldn't cancel the trip entirely. She was fine with it as long as I met my deadlines when I returned.
She should be prepared to take her laptop and get some work finished on her trip. Also be prepared to respond to emails at least twice a week while she's away and call-in if absolutely necessary.