Trevena Mine to the North.
Wheal Ann United to the SW
Wheal Helen to the SE
Not much on the OS maps.
If they want a detailed report done, I have a good chum who does such things.
Without looking at Google Earth, it is quite possible that mining may be more below them than they think, as a very quick comment (rather than a detailed look), if their house is older than 1860, it is unlikely to have a hole under it looking at the various bits and bobs