Learn Arabic (Modern Standard): من هذا الصديق؟

The word for 'who' is من (man). It is spelled exactly the same as the Arabic word for 'from', which is من (min). They are pronounced differently using different short vowels. Since we are asking about a person's identity here, it is من (man).

Where is the word for 'is' in this sentence?

Arabic does not use a word for 'is' or 'are' in the present tense. You simply place the question word and the subject next to each other. The sentence literally reads 'Who this the-friend?'.

Why does the word for friend (الصديق) have 'the' (ال) attached to it?

In Arabic, when you want to say 'this [noun]', you have to add 'the' (ال) to the noun. So 'this friend' is literally 'this the-friend' (هذا الصديق). If you dropped the ال and just said هذا صديق, it would mean 'This is a friend'. To ask 'Who is this friend?', the ال is required.

Does this sentence specify if the friend is a man or a woman?

Yes, it is a male friend. We know this because the sentence uses the masculine word for 'this' (هذا) and the masculine noun for friend (الصديق). If you were asking about a female friend, you would use feminine words: من هذه الصديقة؟.