Since samkhya is a atheistic school, they reject God(Brahman) . But advaita vedanta believe in a God that is Brahman so did any Samkhya philosopher ever refuted advaita vedanta on existence of Brahman?
No they don't. They rather agree with Non Dual singular existence.
Aph. 150. (The Vedantins say, that,) there being a difference in its investments, moreover, multiplicity attaches (seemingly,) to the one (Soul); as is the case with Space, by reason of jars, &c., (which mark out the spaces that they occupy).
Aph. 152. Thus, (i.e., by taking the Sankhya view,) there is no imputation of contradictory conditions to (a Soul p. 170 supposed to be) everywhere present as one (infinitely extended monad).
Aph. 154. There is no opposition to the Scriptures (declaratory) of the non-duality (of Soul); because the reference (in such texts,) is to the genus, (or to Soul in general).