Thank you for your quick reply. Can I attach a doctors note to my dad’s application describing the reason. Thanks
Unfortunately it is mandatory that your father take and pass the medical in order for your mother's application to be approved. There are no exceptions granted and a doctors note will not change this situation.
If your father is unable to take the medical at all, then I'm not sure it makes sense to spend the money to apply since your mother's application is guaranteed to be refused. Even if he is able to take the medical, the chances of refusal due to medical inadmissibility / excessive burden will still be very high. Unfortunately if your father fails the medical then your mother's application will not be approved.
Given your father's situation, it may make more sense not to apply for PR and to spend the money towards a super visa for your mother instead (if she doesn't have one already).
Good luck.