Justin Bieber is being slammed over a “flirty” comment he left on singer Madison Beer’s Instagram post amid fears that his marriage to Hailey Bieber is headed for divorce, RadarOnline.com has learned. Justin and Hailey’s marriage has been under a microscope after his father-in-law, Stephen Baldwin, asked fans to pray for the young couple. Justin’s comment on his longtime friend’s Instagram post didn’t help calm rumors about possible problems in his marriage. On Thursday, Beer shared a series of photos celebrating her 25th birthday, including one of her being surprised with a birthday cake on s…