To be a trainer, you would usually need quite a few years cabin crew experience first. Competition for these vacancies is pretty stiff. If your partner is planning to be crew, then eventually start a family, she would probably end up working in an administrative role (larger airlines have actual "grounded crew departments") once she falls pregnant. This might possibly be in the training centre. After maternity leave she would be back into her flying job unless she actually applied for a ground based position.
Smaller airlines, or airlines with very high turnover may be slightly different. In fact, some have difficulty finding trainers but you would need to evaluate if its worth putting up with bad conditions just on the off chance of being offered a training job.