"You could feel her bones sticking through. She's on the cusp - she looks good now but if she takes it any further, she's going to start to look ill," the Mirror quoted the friend, as saying.
"She's incredibly compulsive. The Spice Girls' reunion is a huge deal for her and she wants to look her very best for her moment back in the limelight. The problem is, Geri is a woman of extremes and she doesn't always know when to stop," she added.
The friend further revealed that Geri's friendly rivalry with band mate Victoria Beckham could be the reason behind her sudden determination to work on her body.
"We mustn't forget that ego plays a major part in all this. Geri loves Victoria, but she also envies her figure and Victoria is looking fabulous at the moment. Geri is always very determined to work on her figure when she knows she's got something to promote and this is her biggest career move in recent years," the friend said.
"That old friendly rivalry with Victoria could well be in full swing again. That's what worries her friends the most," she added.
Meanwhile reports have confirmed that Ginger Spice is dating Russian billionaire Evgeny Lebedev.