Consent letter for divorced parent

[fairoaks]

My divorced sister and her 12 yr old son are applying for B2 visa to visit us in US. Does she need a consent letter from her ex-husband for her son ?

If yes, is there a document other than the consent letter ?
She is not in touch with her ex-husband for a long time and is unable to get the consent letter.

[immihelp]

If she has full custody of daughter and has the documents to prove it, that is all required.