That depends on whether she had a natural birth or a Caesarean section.
As it already mentioned speak with your doctor first. Too, the published guidelines through the ACSM are as follows:
“Exercise in the postpartum period may begin 4-6 weeks after delivery. Deconditioning typically occurs during the initial postpartum period, so women should gradually increase physical activity levels until prepregnancy fitness levels are achieved.”
ACSM Guidelines for Exercise Testing and Prescription – Eighth Edition.