If the owner insist on it, I supose it's her decision. You could persuade her to do a bit of cleaning and test sewing before it's sent to service? Service guys usually don't like basic clean up and polishing (even if that's all it needs LOL). If the manual is still with the machine, it's obvious the owner should have a regular maintantance routine.
I hope all home services weren't as bad as with your mothers machine bkay, so unfair. A customer needs to know everything not to get cheated. It's the same today unfortuately, and we never really do ;- )
40 years on I suppose the grease and wicks would need a clean out anyhow. Companies are stubbern though, only to use "Singer oil" "Singer motor lubricant", original parts. There is something to it though, I like it better when an old machine has all it's original parts and accessories.