Well.........employers do have to tread more carefully where pregnancy is involved, but is this really pregnancy related? The only part relevant to pregnancy seems to be that she took a 5 minute break, which may or may not be pregnancy related? Anyway, I'm not sure the 5 minute break is relevant here.
From what you say it seems that the employer feels that your other half was at least partly to blame for this argument at work. I'm not sure what the maternity issue has to do with this? It just seems to be muddying the water.
By the way, the Government reimburses companies for the maternity pay, so in effect it doesn't cost the company anything (apart from finding cover for her while she's off).
Leaving aside the maternity issue, it comes down to whether the correct procedure was followed for a disciplinary procedure. It's a 3 step process - letter, hearing and then appeal (if necessary). If the employer doesn't follow this procedure exactly then they leave themselves open to finacial penalties. Was this procedure followed?