I think she is no longer angry with you(,) since you've explained in such great detail.
I think she is no longer angry with you(,) because you've explained in such great detail.
Is a comma needed after 'you' in both sentences? I don't think so but my friend thinks otherwise.
As far as I remeber those sentences should be OK. Comma is not needed in neither of them because linking words are already provided. 'Since' and 'because' links the clauses in logical way and putting commas would break the sentences.
[not a teacher]