Hello, my name is Ben and it is my pleasure to assist you with your question today.
In such circumstances the customer should have allowed him to remedy the work, unless there were serious concerns about the safety of the work already undertaken or the remedial work was needed urgently so that getting someone else to do it would have been justified. I cannot see any direct evidence that this was the case so the customer should not have just gone ahead and engaged someone else to remedy the work, charging your friend for this work in the process.
In any event, he would be entitled to be provided with details of how these figures were calculated and why the deductions were made.
At this stage his rights would be as follows:
If there is a problem with recovering money that a party believes is legally due to them, they have the option of pursuing this as a legal debt. The following steps should be followed:
It is also strongly advisable to keep copies of all correspondence in relation to this, in case it is needed at a later stage.