The comma makes this a non-defining relative clause, in which only which is possible.
Without a comma, it becomes a non-defining clause, in which which can be, and often is, replaced by that with no change in meaning.
That can also replace who in defining relative clauses.
I had tea with the friend who/that worked with me in china.
I had tea with my friend, who
thatworked with me in China.
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