Dear Jewant,
The fact that the cause for the left sided weakness is unknown leads to caution ( heart? brain?) so take a lead from the exercises recommended by the physio and modify them slightly so that they are more fluid in a yoga way, encourage the mother to breathe with the exercises so that the baby enters into a joint rhythm with her as this would be beneficial for them both if the mother has been worried about the baby's condition.
At 5 months you are soon going to see whether the effects of this weakness are serious or not as the baby starts preparing to sit and propel herself. At present I would also encourage rolls to the right, which would stimulate the whole left side of the baby's body and also be fun. The diagonal cross stretches also work well as a general way of strengthening right-left coordination.
Mostly, use yoga to diffuse anxiety around the baby, it's great that she has enjoyed her first session with you and continuing from there can only be positive,
all best wishes
Francoise