Apna is neither 'our' nor 'his'. Rather, it always refers to the subject of the sentence. You can think of it as meaning 'his/her/their/our/my/your own'.
Eg: मैं अपना सेब खता हूँ - 'I eat my apple'/'I eat my own apple'
वह अपना सेब खता है - 'He eats his apple'/'He eats his own apple'
हम अपना सेब खाते हैं - 'We eat our apple'/'We eat our own apple'
तुम अपना सेब खाते हो - 'You eat your apple'/'You eat your own apple'
How do we know that Peter is eating his apple and not her apple? I think I understand why it cant be your apple or my apple, but not why it couldnt be hers? The person talking could be pointing at Julia when that person is talking to me. Peter is eating (points at Julia) her apple. ?? Help please?? & thank you in advance