In 2004, the project was brought to a halt when James Hetfield checked into a California rehab facility, where he would spend the next six months, overcoming a drug addiction.
I though it should be there because I think it's a praticiple phrase modifying 'he', but it doesn't seem like it belongs.
Top answer
Personally I would not put it in.
— Feebs11
Personally I would not put it in.
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It should not be there because you are saying that this place, the rehab center, is the place in which he is recovering. If you use a comma, then you're it sounds more like he is simply recovering from a drug addiction.