Is it correct or incorrect to say that fearing someone who is presumed to practice magic is a weakness in one's reliance on Allah?

The default is that a woman is innocent of the accusation of witchcraft, and it is not appropriate to think ill of her unless there is strong evidence. What was said to your brother about his fear stemming from a lack of reliance on Allah is correct, for the stronger one's reliance on Allah, the greater their confidence in Him, and the fear of others vanishes. The servant should recall Allah's power and sufficiency, as stated in His saying: "Those to whom the people said, 'Indeed, the people have gathered against you, so fear them.' But it increased them in faith, and they said, 'Sufficient for us is Allah, and [He is] the best Disposer of affairs.' So they returned with favor from Allah and bounty, no evil having touched them. And they pursued the pleasure of Allah. And Allah is the possessor of great bounty."