According to grammar, the speech in direct speech is as follows:

She said to me, "My father just went/just had gone to the post office when you arrived".

The sentence in reported speech should be as follows:

She told me her father had just gone to the post office when I had arrived.

According to grammar, the speech in direct speech is as follows:

She said to me, "My father just went/just had gone to the post office when you arrived".

The sentence in reported speech should be as follows:

She told me that her father had just gone to the post office when I had arrived.

According to grammar, the speech in direct speech is as follows:

She said to me, "My father just went/just had gone to the post office when you arrived".

The sentence in reported speech should be as follows:

She told me her father had just gone to the post office before I had arrived.

According to grammar, the speech in direct speech is as follows:

She said to me, "My father just went/just had gone to the post office when you arrived".

The sentence in reported speech should be as follows:

She told me her father had just gone to the post office when I had arrived.