They will be asking the witness (under oath) one or more of the following questions:
is this your signature
did you see the deceased sign the document
did you sign immediately afterwards
was the second witness present at the same time
did s/he sign then
were there any obvious signs of duress
or they may ask him to describe what happened, with the aim of answering the above questions in the process
It's not the wording of the will that is under question (if the solicitor is correct) but the procedure by which it was signed and witnessed.