Pregnancy and maternity discrimination doesnt have a minimum service time as long as you were either not pregnant when you joined or told the employer that you were pregnant when you joined.
Her employer was required to perform a new risk assessment as soon as your GF told her that she is pregnant. Two things usually follow, either her role is amended to reduce risk or, where this isn't possible, your GF gets suspended on full pay for the duration.
Also, don't forget that our GF is entitled to reasonable paid time off for antenatal care.
The downside is that while it does appear that the boss is being at least a bit of a tool, proving it is....difficult. Tell your GF to keep records of all such goings on in work, especially that 'disciplinary' (which I would note in detail since it is actually utter BS) which appears to have been performed incorrectly and solely as a result of announcing the pregnancy.