To be able to dismiss her immediately the employer has to shoe that there was an offence amounting to gross misconduct. As she has more than 2 years’ service she is protected against unfair dismissal so the employer will have to hoe that there was a fair reason for dismissal and also that they had followed a fair procedure.
What makes smoking a gross misconduct offence will depend on various factors. For example, was there a clearly communicated policy which said that smoking is prohibited in no-smoking areas and that doing so would amount to gross misconduct? Would smoking where she did expose others to risk or affect the image of the company, such as if she is seen by service users, suppliers, clients, etc?
There has been case law which has ruled that smoking in non designated areas, even if it was a one off offence, can amount to a fair dismissal. You can read about this here:
It does not automatically mean it can amount to dismissal but if there was a clear policy which said that breach of the smoking rules can amount to dismissal then it can happen.
This is your basic legal position. I have more detailed advice for you in terms of the options she has on taking he matter further if she is dismissed, which I wish to discuss so please take a second to leave a positive rating for the service so far (by selecting 3, 4 or 5 stars) and I can continue with that and answer any further questions you may have. Don’t worry, there is no extra cost and leaving a rating will not close the question and we can continue this discussion. Thank you