1. Allah is the Creator and Sustainer of all things. He created all that there is. All that exists. The jinn, the devil etc. all exist as per the will and decision of Allah. No evil force has any power against Allah and can do no harm to Allah.
Before Prohpet Adam was created, Iblis/the Devil was a 'good' worshipper of Allah. He had access to the heavens and was in direct contact with Allah. However, when Allah created Adam, Iblis was asked to prostrate to Adam but out of his arrogance, he refused. Instead of crushing and destroying Iblis there and then, Allah gave him respite, meaning allowed him to live for some time. Iblis made a promise to misguide the children of Adam. Only the obedient, faithful ones are saved from the misguidance of iblis. Iblis has a family and massive tribe of jinns out there who solely aim to misguide humanity. Allah allows it as part of a test for all mankind.
After the day of judgement, Iblis will be thrown into eternal punishment and so will all the people he managed to corrupt and misguide.
Our mission is to pass the test set out for us. Of course Allah could destroy Iblis, destroy all evil but then there would not be much of a test.
2. Allah sent around 124, 000 Prophets and Messengers over the course of time. The bottom line is, it was the will of Allah. Just as he willed the sky to appear blue instead of pink and the grass green instead of purple and the number of fingers on your body 5 instead of 18 etc. there's a billion such examples where Allah willed something that we can ask why it wasn't different. If your friend is going to ask about this issue, he needs to be consistent and ask about all these other billion things too.
3. The Quraan. Tell your friend to learn Arabic and witness the miracle of it. At the time the Prophet gave his message there were many such poets out there who were well regarded as being elequont but they were dumbstruck as to the source of the Quraan due to its magnificient and amazing nature. There could only be one source, Allah.
4. There's many articles which detail slavery. Many threads even here too. Please take the time to search through the forum.
5. This world and everything in it is worth nothing in comparison to the beauty of the hereafter. It's like a drop in an infinite ocean. Our purpose is to pass the test by living according to the will and commandments of Allah in order to be granted paradise. We need to show submission once we've recognised that Allah is in control and command. Only an arrogant person would find it difficult to do this.
6. Again, the will of Allah. This question, however, shows that the person asking it is lazy. It's like saying 'why must I do things to prove my love to my wife (e.g. buy flowers, look after her when she is ill etc), when it would be easier to say I love you only?'. It takes nothing to just claim love or verbally say something but action shows that you really mean what you say.
7. Another topic that has been discussed many times here. Please search.
8. We don't know what is on those planets. However, this does not mean there is no purpose behind their existence. We shouldn't be so egocentric and assume it is all about us, to the extent that we question the wisdom of the existence of planets we can't access.
9. Neither affirms nor denies their existence.
10. Comes back to the issue of life being a test.
If I may advise, it's very easy to ask 'why this', 'why that' but at the end of the day, your friend needs to question himself on the basic fundamental things:
i. Does God exist
ii. Is Muhammad, pbuh, a Prophet of God
If your friend can find it in him to believe in the above, then he has no choice but to accept that some things he just cannot know the reasoning for but must simply obey out of humility and submission. He owes it to himself to cast aside his arrogance and acknowledge his limitations.
hew
because I have a lot of Why questions about his question 
this is just a test lets say a candy before the open buffet