To me it sounds as if there are more issues here than are written.
It is debatable whether somebody can be sacked for calling someone 'a vindictive *****'- certainly in my previous workplaces, both parties would have been suspended and persons interviewed separately and then the decision would be taken as to what the correct step would be to take. Depending whether there have been previous instances within certain time periods, and whether anything was still 'live on Employee's files would determine the action taken against the Employee.
Calling somebody a name, especially if it does not fall within sexual or racial discrimination is not necessarily enough under a gross misconduct.
Missing an interview, can warrant a decision to made without the interview, if the Empolyer wan't notified of non attendance by the person concerned.
Weighing the facts up that are listed, I don't think your Mum necessarily has the upper hand. I wish that she did, but, the Employee has also got to ensure that they fall within H&S policiies, and dignitys at work etc. Under H&S policies, your mum could be seen as failing them if she did notify HR or Management of troubles with diabetic control that could lead to other issues. Under dignity at work, is it reasonable to expect that an Employee does not call another employee 'a vindictive *****'?-whether suffering high BG's or mood swings-then I would not expect my previous staff top to have this sort of behaviour....
To be quite honest, I agree that you Mum should be taking advice-from CAB-or house insurance legal helplines and any other sources that have been listed. I actuallt believe that there is more to this than just the facts listed......but then I have been used to previously ripping statements apart that staff have made, and have had people walk out with me interviewing them because they know they have been caught out giving inacccurate details.
I'm not saying that details here are inaccurate or anything such like, but I believe that your Mum should be taking this up as an issue asap and getting help from any source that you can get help from.
Most Companies do not taking the sacking of a person lightly these days, as tribunals are a true reality. I do not know the size of the Company that you and your Mum work for, whether they have a HR departement etc. Some Companies are still unscrupulous in this day and age, but just in my own opinion here, the facts listed don't appear to be complete.
I really hope that you manage to get some legal help.