My 36 Year Old Sister Had Surgery To Repair A

My 36 year old sister had surgery to repair a torn ACL (patella graft) yesterday. The surgery went well. However twice today she has fainted when trying to stand on the crutches. The second time, we spent 10 minutes slowly sitting her up to acclimate her, prior to moving her. She fainted with her eyes open and was unresponsive for roughly 30-40 seconds. Slowly she came to.

Is this normal? Are there any things we should be doing differently?