No, like you I've heard the opposite, that Henna makes hair thicker and that it can relax curl (which might be where she got the idea of 'thinning')
Sounds to me like either your friend felt like she couldn't back down or admit she might have been wrong about henna thinning or else she was comparing your still wet hair to dry as you said.
Personally I certainly haven't noticed any thinning and since I'm a border line ii heading for i I think I'd notice. I wouldn't let it bother you especially if you haven't noticed a difference.
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