Whats the difference whether the descimination occurred during the interview, or after her hiring?
There's a big difference ... some people give up one perfectly secure position to be hired for another.
Do you believe that it's OK for an employer to maliciously uproot an individual simply because he/she smokes?
Ethically or morally, probably none. However legally there could be a big difference. You have no right to be hired, but once hired you have a legal interest in keeping your job. At least under US law, in at least some states, but I wouldn't know about English law for certain.
Discrimination is a good thing.
The difference is that she accepted employment, thereby terminating her search for employment with another company. They did not tell her that not smoking was a condition of employment. They could just have easily asked her if she wore a thong and fired her for that. The question should have been asked prior to the offer of employment, and prior to her acceptance of such.
The interview phase is the time for both parties to determine that the deal will meet any legitimate requirements they may have. Would you leave salary negotiations till after starting? Once an offer has been made and accepted, no further significant issues should come to light. As a practical example of why this is so, she may have resigned from another position to take this job, which she obviously would not have done if they made clear during the interview they weren't interested in hiring smokers. She almost certainly has lost time in her job search due to believing she had a job.