Next, has there been anything written down before about the employee not being able to undertake other COACHING ACTIVITIES as an extra to the employment? I do not believe an employer can demand that "it is not acceptable to engage in any other business, activity or employment (either inside or outside your normal working hours) which interferes with this duty" - particularly as this is a 25 hours contract - it is an unreasonable constraint on the employee. But I suggest she keeps her powder dry at present by not challenging it until she has more evidence for the employer seems determined to do, to tie her planning into knots. What the employer can demand is that she does not undertake self-employment at the same centre-and I am unclear whether that has happened..
Finally regarding the potential for unfair dismissal, some basics:
Has she been there at least 12 months continuously?
Do you realise that constructive dismissal means that the employee has to resign first, then claim (and prove) constructive dismissal at an ET? The risk is on the employee.