If I understand correct, she has valid VISA ( stamp in passport) and I 94 till Sep 24th. (Please don’t travel to Mexico without Valid US VISA / Canadian VISA / Mexican VISA / European VISA). I suggest travel bit early, do not wait till last week. Most probably it will work out, it worked for all the people which I knew, but always there is 20 - 25% risk which we should accept. Means she will be allowed in to USA, but the getting I-94 extended is purely depends on the CBP officer. She should explain in case if officer raises concerns. Please take the printouts of Automatic Visa revalidation page from Travel website.
All the very best.