It really depends on company policy. Unless there is a stated policy that the company may monitor Email, and unless (as you mention) there is a condition where secretariies filter Email for their bosses, there is every expectation that "private" email will remain reasonably private, ("Reasonably" means given the realities of computers and technology.)

I suppose things may be different in Britain.

In any case, the lady in question had already resigned from her job. What demonstrable harm was being done? It wasn't as if the lawyer(s) iinvolved were actively discriminating against her on the job.

It was a stupid thing to Email, no doubt, but I don't quite see it as lawsuit fodder... but of course the lawsharks rise to such a thing like Floriida sharks riise to swimmers.