Janaazah (funeral) prayer behind a Qadiani (Ahmadi)

Assalaamu 'Alaikum,
I attended a funeral today for my wife's aunt. After the janazah prayer, I found out that the person performing the janazah prayer was not muslim. He was an Ahmadi belonging to the sect who says Mirza Gulam was a prophet. I personally feel that the janazah was not valid, but there were a lot of muslims in the congregation, who paid little attention to the fact that the leader was not muslim. Should my family and I re-pray the janazah?