Hello, Rosa. I'm sorry to hear that your dad is having problems with his TWA. I've reached out to you via email to help you resolve the issue.
I had this same exact problem where it worked for a few days and then stopped outputting sound. The solution was to replace the actual battery that goes inside my acoustic guitar. My guitar had the battery located right on the input jack. Once I did that it works like a charm again! Make sure to unplug the Tonewood amp from your guitar when you aren't using it as it will constantly draw power. It seems the tonewood amp simply drew power from my guitar preamp for a few days strained and the battery died. Here is a quoted tip from this site that I found to get the solution:
""Saving your guitar battery. Active pre-amps (a guitar with a battery) are always on until the 1/4" plug is disconnected from the guitar itself. Please disconnect the short cable from the guitar when the TWA is not in use.""