My husband polishes his nails too. No harm done. Now he has moved on to polishing brasses😀 Every brass ornament he can find he polishes them to perfection. Then starts again. I put some of his favourite music on while he is doing it. Gives me a little time to sit and read.
He also has started humming all the time. Unfortunately nothing that could be recognised. Family and friends are aware of it and OK about it but I worry it may irritate others when we are out and about.
We have a friends who come occasionally. Glen goes out for a walk with my Alan and I take Joan who has dementia out for lunch. When we are all back we have tea and a chat.
Alan was humming away and 4or 5 times Joan said 'What's that noise?' To which I replied Its Alan humming. Then eventually she asked her husband and when he said 'it's ok Joan, it's just Alan humming' she said ' Will you tell him to stop, Glen'
I'm afraid Glen and I found it quite funny.
But what I learned was that Alan is not aware that he is humming. He denied he was humming. Said it must be the wind!!
I know it's hard to keep a sense of humour when we are going through this but it helps if you can.

What a pain, anything so repetitive grinds one down eventually. It's an obsession, maybe something she enjoyed in the past. She probably doesn't even know she's doing it some of the time. My Mother's brain switches off, she does things sometime and then accuses me of doing it.

Oh, may God Bless YOU! Second hand smoke is horrible, not least because you don’t smoke and someone else is doing it to you. My husband was able to quit, after 60 years, by accident really! Haha. He had seizures and suffered from anxiety attacks. While in hospital one time, they added a anti anxiety drug called Busiperone to his list of meds.
And he simply quit! I am everlastingly proud of him. He had already gone through radiation for throat cancer! Still smoked for 3 years after that. So, my point is, nothing works unless they decide to. Maybe slip buspar in her coffee? Lol. Good luck

Humming sounds quite a mild thing to do. Your friend should try living with someone with Autism. Constant squealing and Wiggles or Abba music. That's on a good day. One has to laugh. 😅😁

We all have our comfort habits. My Autistic son has an ability to make lots of things into obsessions. Repetition day in day out. Got used to it now.
I like a glass of wine, my Husband plays golf watches golf on TV if it's not golf it's football. With some it's food.