Sounds similar to the junkmail I get from domain registrars that are NOT who I do business with, "notifying" me to renew my domains through them for 3-5X what my registrar Namecheap charges - and considering I pay Namecheap an extra $2/year/each for "WhoisGuard" to obscure my personal registration information, it's rather puzzling as well as annoying.

I'd suggest double-checking with the company who DOES do his hosting to make sure his account is active and current and his host hasn't been acquired by anyone else, and if so, you can safely ignore this "invoice". Then, if he gets a "second notice", you may want to contact Datamine by phone and ask them what the "service dates" are to see if they're claiming any existing contract or just trawling for fishes.