You would likely need to buy the same non-working Sonos One SL, or the necessary parts, if you can find them online and go from there, maybe using a YouTube teardown video for example, like the ones seen in this LINK.

Personally speaking I would sell the damaged speaker on for spares, and then purchase a new or refurbished one from Sonos, maybe using any trade-up discount for a new product purchase (if your Dad has that available to him in his online Sonos account). 

To cut your own costs, maybe raffle a bottle of whisky, or something similar, between all your family members and friends and get them to ‘kindly’ assist with the purchase that way.

Also see this link:

https://www.sonos.com/pl-pl/shop/certified-refurbished