If she is working a standard 9-5, this is a 7 hour working day, = 1820 hours a year which means her hourly rate is €27.47 and at a standard one and a half times rate for overtime, this means her OT rate is €41.2
Therefore is she earns €20k per annum OT, she is working approx 9-10 hours overtime per week
OT should not be compulsary in any job, and I have known people who became financially dependent on it and struggled when it was stopped. Therefore by increasing her salary she gets more certainty for the future, it may improve what her employers pay into her pension(if anything) and may help in terms of mortgage applications etc going forward. I'd suggest a minimum of €10k to split the difference and show willing.
However if she is at her desk until 7pm every night, I'd also suggest it's time to get the CV out. Life is too short