When the woman at the museum asks Stephen if they're still on for Friday at the "best steak house in town" and Stephen's confused, the woman wasn't surprised at all at Stephen's British accent when he engages in conversation.

Marc has an American accent and, while he could have faked a British one temporarily, we now know Marc is only pushing Layla away (divorce papers, refusing to take her calls) to keep her from being Khonshu's next avatar. He still seems to love her and probably wouldn't cheat.

Which means it probably wasn't Marc who asked the woman at the museum out. It had to be another identity that ALSO has a British accent.