Only ever needed ID to get a beer.
Yes it can recognize you by fingerprint
I never have had to show or use my ID to get into six flags so your friend should be fine entering
If you are using season pass they usually have you use the fingerprint that was registered with the pass. I haven't had to use my ID ever.
Most of the time they dont check id as long as u have your pass. I have had my ID checked once or twice
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