I've no reason to doubt that it is from Pratchett, although checking on the web reveals only the same quote similarly attributed, without context - and, as we know, sometimes self-perpetuating mistakes get made. [ETA: Thanks to sovay, I now know it's from Thud!]
A bit of context would be nice, though, since I really can't see what this quote means. The only way I can make sense of it is as a warning that drinking coffee shortens your life - but in that case, why would a cafe advertise the fact? (This isn't a Gothy, half-in-love-with-easeful-death type establishment, but a cheerful place full of mums and toddlers, which sells Pom Pom Bears and Tunnock's Tea Cakes and milk shakes made from Maltesers. It may be called Lashings, but you just know they mean ginger beer rather than BDSM.)
So, what coffee-friendly meaning might this Pratchett quotation actually have? I'm quite prepared to believe I'm missing the bleeding obvious.