An interesting read about the use of a 'non-expert' to provide evidence on the authorship of handwriting and signatures.
In this case, Judge Perry has the right to accept the testimony of a person who has extensive exposure during their business dealings to the handwriting of the person and documents in question.
Equally so, the defense lawyer has the right to challenge the use of the evidence, however this would need to be done on the right basis. Handwriting can be deceptive, there are many reasons that it may look similar or different to that expected. Without a true evaluation of these reasons, there can be a danger of wrongful evidence being innocently introduced.
I'm sure the Judge was accepting the evidence cautiously.