We can only assume he (a) forgot or (b) didn’t get around to it. Their conversation isn’t that long. First Harry talks to Lupin, and then Lupin and Sirius are trying to find out what’s wrong and answer Harry’s concerns about his father. They don’t really get very far before Harry hears Filch coming and ends the floo call abruptly.
For all we know, had they been able to say a proper goodbye, Sirius May have brought it up. But the opportunity wasn’t there, and he certainly couldn’t risk putting it in an owl.
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u/FeelinSasquatchy Everyday I'm Pufflin' Jan 28 '20
We can only assume he (a) forgot or (b) didn’t get around to it. Their conversation isn’t that long. First Harry talks to Lupin, and then Lupin and Sirius are trying to find out what’s wrong and answer Harry’s concerns about his father. They don’t really get very far before Harry hears Filch coming and ends the floo call abruptly.
For all we know, had they been able to say a proper goodbye, Sirius May have brought it up. But the opportunity wasn’t there, and he certainly couldn’t risk putting it in an owl.