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Hi!

Did you know that Mrs. Cameron is good friends with your grandmother?

The above sentence, shouldn't it be like , "Did you know that Mrs. Cameron is a good friend with your grandmother?" ?

It's hard to understand how come, "is good friends with somebody" is possible.
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PructusHi!

Did you know that Mrs. Cameron is good friends with your grandmother?

The above sentence, shouldn't it be like , "Did you know that Mrs. Cameron is a good friend with your grandmother?" ?

It's hard to understand how come, "is good friends with somebody" is possible.

Actually, it's quite common
Philip
Pructus
Did you know that Mrs. Cameron is good friends with your grandmother?

Shouldn't it be "Did you know that Mrs. Cameron is a good friend of your grandmother" ?
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No, good friends is a common expression. I suppose because they are each friends with each other, giving you two friends, so that is why it is plural. It is talking about the relationship going both ways.

If you were to say 'Mrs Cameron is a good friend of Peter', it could be possible that Peter is not a good friend of Mrs Cameron. The friendship may mean a lot more to Peter than Mrs Cameron. When you say 'good friends' then it includes both of their perspectives.
I see...

Thanks, Philip and Nona....

pructus
Thanks, Nona.
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Did you know that Mrs. Cameron is good friends with your grandmother?

'Mrs.' is AmE, while 'Mrs' is BrE.

So in BrE, it should be "Did you know that Mrs Cameron is good friends with your grandmother?"

Am I right in saying so?

Thanks in advance.
It doesn't really matter. Some people use the period and some people don't.

The only time you should absolutely avoid it is in addresses (and only because our post office had a campaign to wipe out the use of punctuation in addresses to avoid errors caused by scruffy handwriting!).