I wouldn’t be too quick to blame the stringer.
Without photos to make a better judgement, whenever I hear that a string (mains) break near the top of the frame, that is usually the result of a mishit.
This suggests to me user error and not the fault of the stringer. When you mishit, there’s always the possibility the string will break.
However, if the string is breaking at the top where the finishing knot (A5) usually is then it’s possible that it’s how the knot was tied-off. I know some Yonex stringers are moving the tie-off knot from a5 to lower shared grommet for the crosses for higher tension jobs.
In the future I’d request the specific stringer you want amongst the 3 for your racket (this may cause you to wait longer to get the racket done).
Other things to consider, check the state of your grommets around the top half of your racket. Are they in good condition or are parts of the frame exposed? Does the stringer replace grommets (if they dont….RUN AWAY)?
These would be all the things i’d be checking for both as a client and as a stringer of rackets.
Simply asking questions of your stringer will help you get an idea of the level of detail and care that is put into stringing a racket.
Hope this helps