Yes, a lot of folks who end up in AA are "mad at God" and call themselves atheists.
You can't be mad at God and be an atheist .... God exists, you're mad at Him !
My Higher Power that I call God has "evolved" over the years so I don't think an accurate and absolute definition is necessary nor possible.
Tell the newcomer to try to keep an open mind and it will all make more sense down the line.
All the best.
Bob R