I will take your two questions in reverse order for reasons which will hopefully become obvious.
is it worth presenting our case to the employer before he dismisses her, to prevent it ever going to tribunal in the first place?
Quite possibly, but get advice. This is something you would need to talk to a specialist with about with specifics, but it is very often advisable to address serious issues through a formal complaint (known as a grievance), made in writing and without undue delay. If you do later want to rely on the fact that you had raised an issue, a tribunal may not take you seriously unless you had exhausted the internal channels (i.e. raised a grievance and appealed if the result was not in your favour).
Obviously we will consult an employment law specialist, but is there any point in doing this before the employer takes final action?
Absolutely, yes. You ought to get help deciding whether to raise a grievance, preparing and presenting your complaint, and support with any tribunal proceedings which arise - as there are strict time limits (for a typical tribunal claim, three months less a day from dismissal) and steps which must be followed to the letter or your case must be thrown out.
If your friend had the sense to join a trade union before this started, they should contact their rep immediately: this sort of thing is their day-to-day work and they will provide not only advice but also would normally accompany you to a grievance hearing or any meetings which might result in your dismissal (e.g. capability/disciplinary) and, if you have a good case, support through the tribunal process, and some unions will also help with the upfront cost of tribunals. You should get them involved as early as possible as there are things you can do - or fail to do - which will significantly affect your chances if it ever comes to tribunal, and also your employer's willingness to either back down before they make rash decisions, or at least settle before it gets to court.
If your friend is not a union member, getting proper advice from a solicitor who specialises in employment law might get expensive - but it might be worth it, depending very much on your circumstances (e.g. how much the job pays, how easy it is to get another one, the details of the case). In terms of solicitor, some offer free consultations - but only for small amounts of time, too small for serious work - but enough to help you decide whether to hire them to get advice. Look for one who specialises in representing employees and not employers. I have also heard that the Citizens' Advice Bureau sometimes gives employment advice, which might be a cheaper option - but they won't be able to represent you in your grievance meeting or in a tribunal.