Bush posed for a picture with Huma Abedin, the longtime Clinton aide who was hosting the fundraiser with Vogue editor-in-chief Anna Wintour.
The photo was then posted by multiple event attendees, including Lauren Santo Domingo, co-founder of the online fashion website Moda Operandi.
A spokesman for the former president did not respond to request for comment. Clinton aides also did not immediately respond to requests.
Barbara Bush is the latest member of Republican family to tip their hand toward Clinton over Republican nominee Donald Trump.