First of all, congratulations!
The only advantage of having an employment contract is if there was something promised at interview above and beyond normal terms and conditions.
Even if she does not have a contract, if there is a dispute, she will be in a fairly good position as her word would be believed ahead of her employer, as he has been negligent in not giving her a contract.
Most companies do not pay maternity leave so she should not expect this.
If it is her company's policy to pay maternity leave, then not having a contract won't disadvantage her. She can claim it as part of the general practice of the company.
It is a good general idea to notify your employer as soon as possible. In the unlikely event that your employer is considering any sort of disciplinary action or changes, he will be much more reluctant to do it once she has notified him that she is pregnant.