Many examples of immigration offices refusing to correct error of stamp from entry to Thai airports.
No so much in Bangkok where you can attend immigration to have error corrected.
Not sure current office for this however previously was counter K at IT mall.
Certainly the stamp can be altered at BKK airport.
For the OP, all this is very inconvenient.
Obviously she will attend Samui immigration Monday.
All the TM30 chit chat is useless.
My guess is Samui will tell her to nick off.
Fall back option is to change plan to achieve 150 days using setv entry.
That's possible without having stamp corrected