WhatsApp's group management feature works in a way that once a participant voluntarily leaves a group, they cannot be directly re-added by any member of the group (including administrators). This design is intentional and has some reasons behind it:
Respecting user privacy and choice: When someone decides to leave a group, it is considered their voluntary action. Re-adding them without their consent could be intrusive and may violate their privacy preferences.
Preventing spam and harassment: Allowing members to re-add someone who has left could lead to potential abuse, spam, or harassment. People might repeatedly add others who wish to stay out of a group, causing annoyance and discomfort.
Avoiding group conflicts: If a member leaves a group due to some disagreement or conflict with other members, re-adding them might not be conducive to a positive group environment.
Managing group dynamics: When someone leaves a group, it may indicate that they are not interested in participating or have lost interest in the group's topic or purpose. Allowing them to re-join might not align with the group's objectives or dynamics.
However, WhatsApp does allow users to join a group through an invite link, even after they have left the group. If you still wish to include your friend back in the group, you can share the group's invite link with her, and she can decide whether to rejoin or not.
Please note that WhatsApp's features and policies might change over time, so it's a good idea to check the app's official documentation or support channels for the most up-to-date information.