my daughter is spending the summer in Thailand, around 45 days.  We had planned to spend time visiting Angkor so the extended stay was not an issue until we got covid and cancelled the Angkor trip.

Now we plan to visit some islands, but i would like to know how early i can extend her trip before we leave.

It's up to the immigration office. I have entered visa exempt and applied for extension first day and got it (Jomtien). I put reason as wanting to do 6 week Thai language course and needed extension before I book, but they didn't really care.

 

All you can do is go to immigration and see what they say

I would say the effort (and disruption of holiday) of extending a visa here is higher than applying for a proper 2-month visa back home. 

It depends on where and who. I have managed to extend it one week into my exemption stay ,I think it was in Rayong, and in Pattaya one IO screamed at me too early, come back in 2 to 3 weeks. 

Bear in mind that, while you are staying in hotels, the local immigration office to the hotel can do the extension of a visa exempt entry. Maybe, just do the extension while on one of the larger islands with an immigration office.

It is important for your daughter to check specifically that she is staying at a hotel which has provided the local immigration office at which she will be seeking the 30-day extension with the notification needed to comply with TM30 requirements. Without such a notification her chances of obtaining such an extension at that particular office will probably be zero.