What are the requirements for opening a bank account for a guardian visa in Thailand?

It depends on where she applies for her initial Non-O visa based on a parent of a foreign child.

IF she applies for her visa at the local immigration office INSIDE Thailand:

She needs to have funds in a Thai bank account when she applies, means she has to have it in advance.

IF she applies for an initial visa at the Royal Thai Embassy or Consulate-General in another country: She only needs to have funds in a Thai bank account for a month (for 1st year) when she applies for an extension at the local immigration office, means she can have it just after getting an initial visa.

The funds need to be in the thai bank account in her name only before she applies for the 90 day Non-O visa based on supporting a foreign child legally enrolled in a school on an ED visa here.

They won't accept the application for the 90 day Non-O without the proof of funds too

When you apply for the year extension for the first year the funds need to be in the account for 30 days before the extension application and for every subsequent yearly extension the funds need to be in the account for 3 months before the application

𝗜 𝗿𝗲𝗮𝗱 𝘁𝗵𝗲 𝗿𝗲𝗰𝗲𝗻𝘁 𝗱𝗶𝗳𝗳𝗶𝗰𝘂𝗹𝘁𝗶𝗲𝘀 𝗳𝗼𝗿 𝗼𝗽𝗲𝗻𝗶𝗻𝗴 𝗯𝗮𝗻𝗸 𝗮𝗰𝗰𝗼𝘂𝗻𝘁𝘀 𝗼𝗻 𝘀𝗵𝗼𝗿𝘁𝗲𝗿 𝘁𝗲𝗿𝗺 𝘃𝗶𝘀𝗮𝘀.

That may well be but this isn't the group to discuss opening a bank account in thailand.

Post on a group related to where you will stay in thailand and ask for suggestions because your wife will definitely need a thai bank account in her name only with 500K in it before she can apply for that Non-O