As a rule, whippets are friendly with strangers, often even if not introduced! Whippet owners even have a name for this never-meets-a-stranger personality trait. We call it "excessive greeting disorder" and warn people to be ready for a two minute whippet-intensive encounter when they arrive at the house.
Whippets are rarely much good as watchdogs. As a rule, this is not a very vocal breed, and some hardly ever bark at all. (There are exceptions.) Whippets are too small to be useful for protection. If you need a watchdog, get a Doberman!