Friendship can be expressed in a relation between to persons. Thus a association between them. Basically I would model the domain model something like this. It can be more complex, depending on goals. But its a start:
Never directly use the account/user entity for this, these are mend for account handling. I work with a association between Account and a new entity Person. This person can invite another person to become a friend. This friend can accept this. the associations and referenced object can be created and set using microflows in the invite process.
I suggest you go to https://gettingstarted.mendixcloud.com/link/path/10 and follow the steps to learn the basics of Mendix.