What should my mum do to avoid overstaying her visa exemption in Thailand?

45 days a thing of the past only 30 days now surely you live here would know that ?

Based on what will she get 45 days visa exemption?!

With Spanish passport you’re entitled to 30 days visa exemption. The extension is only 30 days again. (45 days during Covid). So this gives up to 60 days in total. I would recommend that your mum apply for a 60 days tourist visa at the Thai embassy in Madrid before she leave and extend this with 30 days at the local immigration in Thailand. Then you’re safe and there’s no overstay.

They use E-visa so you apply online with a personal user. -visa?menu=5d7cf78c15e39c3e99d

How she has granted 45 days visa exception? I thought it was 30 days

You're incorrect. There is no 45 day stamp. If she arrives without a visa she will get a 30 day stamp and can apply one time for a 30 day extension. That's 59 days in total since the day of arrival is counted as day 1.

That's not sufficient time based on what you've said. She would overstay her permission to be in Thailand. And her airline might not let her board her flight without proof of a ticket out of Thailand win 30 days.

She should apply for a tourist visa before traveling which will give her 60 days on arrival and can apply one time for a 30 day extension.