Well he did ask, so it is not as if you are imposing your knitting on the family!
I would knit him the scarf but maybe keep it a fairly simple pattern, just in case she is the sort who thinks that anything fancy would be you deliberately trying to show her up!
Or you could use this as a way of getting to know her. Maybe a phone call such as 'Kevin says you knot scarves, and i would like to knit him one, do you have any tips or patterns you could recommend?' Nice way to get to know her, and (again if she is the sort) it keeps her involved so she can't think you are taking it away from her.
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