You should indicate in the letter that your friend has your and your hu*****and's authorization to travel with your child to US. This letter should be signed by both of you and your hu*****and and also need to be notarized.
Sometimes, the US officer at Custom might not require the letter. However, if require, your friend should have the letter.
Sometimes, the US officer at Custom might not require the letter. However, if require, your friend should have the letter.