Girl's Behavior

My friends behaviour is worrying me Is this a sign of mental illness?

So my friend is absolutely obsessed with trees and plants- to the point where I’ve had to stop her from climbing fences and going onto private land to get photos of trees she likes.
sometimes she will just randomly disappear and sit in the woods alone.
She also talks endlessly about things that are plant or gardening or tree related as if she doesn’t understand that it’s a little excessive and strange to talk to for example- a shop assistant about all this.
She also talks to herself under her breath, only really talks to me or our one other friend about anything. Also she has some hygiene issues. She doesn’t smell but her hair is usually pretty unwashed and she does have decaying teeth. All of this aside she is VERY intelligent. Not at all what this description might make her sound like. If I ask her if she’s okay she says she is happy and fine but she is very blunt and monotone in her manner of speaking.

My friends behaviour is worrying me Is this a sign of mental illness?
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