Is there any "logic" behind such an unusual construct ( = IS is followed by a noun in the plural)?
Perhaps there are other English expressions/constructs following the same pattern, i.e. "Person A is <noun in the plural> with Person B"? e.g. "Nick is enemies with Bob" or something :-)
The given sentence is okay. This is an idiomatic-type construction and therefore does not need to be strictly grammatical. The meaning is (here's where a kind of logic behind the construction can be seen): Sally and Tom are friends.
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The given sentence is okay. This is an idiomatic-type construction and therefore does not need to be strictly grammatical. The meaning is (here's where a kind of logic behind the construction can be seen): Sally and Tom are friends. Other constructions similar to this are, for example:
Nick is enemies with Bob. (= Nick and Bob are enemies)
I'm enemies with John. (= John