Bouncy Brick is Southern England, Van-B is Southern Scotland (iirc)
in any-case both are UK Residents (and very likely Citizens too)
As far as this going, while it's legal for them to check e-mail going in/out if the message is personal then it cannot be used against them. As personal messages are protected under DPA.
Now as far as sending a personal message, while this is frowned upon there are no legal routes that someone can be forced to resign without the internal e-mail system being abused.
If she was Personal Messaging all day, or Sending out high-volumes of Personal E-Mail. The company would have a problem, and if they have a problem the Employee's got a problem.
They cannot force a resignation based on a Personal Message though. Technically they are not allowed to read it once they realise it isn't company related.
Now there are some recent cases of people being fired because of misconduct by slagging off thier current employers. This isn't common though, and in order for the claims to be valid it has to be proven that the companies reputation can be damaged to a reasonable extent.
For example if your bitching to your friends at home about work, that's likely to reach 10 maybe 20 people at best. On the internet your web-blogs *might* reach potencial millions. I believe the currently accepted cut-off point is 10,000 hits per week.
Sounds a lot but isn't. So it's all within a 'reasonable doubt' scenario.
Personally I would suggest against out-right legal action, as this can often lead to more costs than worth. However what I would suggest is when she applies for her Serverance Package she talks to her Solicitor first, and have them draw up a legal document stating that she had an unfair dismisal.
Often the hint that there could be a costly legal battle is more than enough to negotiate a reasonable severance package. As 'K' has already resigned this won't affect her directly per'say however previous referenced used from that employer might. Again part of the severance package could be that they acknowlage the unfair dismisal.
This will show in a better light than a forced resignation for a next prospective employer.
Damage control and providing enough cash to survive on in the mean time, is really the order of the day.
Just remember to let her know that friendly calm hints will provide her with more than some blindly angry 'I'm going to sue the pants off of you' action.