There's nothing in law that says a dismissed employee can't contact colleagues who still work there (just do it outside of works premises and company time). But I would question what she is hoping to achieve. These things are usually best kept between the employee and the company until all over.
Gross misconduct means a breach (of contract) regarded as so fundamental as to mean the employer decides to dismiss the employee without notice.
She should use the appeal route with the employer first.
Assuming she has been there more than 12 months continuously, she could then consider Employment Tribunal.
It is impossible to say whether these circumstances justify what happened. But misuse of company property - particualry in circumstances that could have led to further destruction of company property (viruses) is likely to be regarded as very serious - so I am not surprised.