Whether we remain on the right path or go astray is entirely...
Whether we remain on the right path or go astray is entirely our own choice and it is our own free will. Fifthly, it is only when man, by his own will and choice goes along the wrong path that Satan gets him in his grip. Allah says in the Qur'an (7:27): "Verily, We have made the Satans guardian of those who do not believe." Notice that when one becomes unbeliever he comes under the domain of Shaitan.
Sixthly, it should be pointed out that in this life we have been given equal options of doing good or evil. And to keep the balance, we have been provided with one invisible enemy (Satan) and one invisible guide (intellect); and also one visible enemy (human Shaitan) and one visible guide (Prophet and Imams). So, you cannot say that balance is weighted towards evil. Finally, it should be clarified that in your question "man was created weak" has been quoted out of context.
It is not meant to compare man with Shaitan and it does not declare that man is "weaker than Shaitan". The Aya should be seen in its context. It runs as follows:- "Verily, Allah desires to turn unto you (with mercy); and those who follow their own lust wish that you should deviate a great deviation. Allah desires to lighten your burden and man has been created weak." (4:27-28) This Aya depicts the Mercy of Allah upon mankind because man has been created weak.
Is it not ironical that your friend wants to use it to prove "injustice" of Allah? Q3: WAS SATAN AN ANGEL? According to Holy Quran, Allah told the angels to bow down to Hadhrat 'Adam; everybody did except Iblis. Was Iblis an angel or jinn? A. Here are 3 ayat which provide the answer- "And when we said to the angels, "Bow down to Adam"; so they bowed down except Iblis.
He was from the Jinns, and he broke the command of his Lord." (Qur'an, 18:50) "He created man from sounding clay like unto pottery, and created Jinns from smokeless fire." (Qur'an, 55:14-15) "Said He, "What prevented thee that thou did not bow down when I did command thee?" Said (Shaitan), "I am better than him; Thou hast created me of fire and Thou created him (Adam) of clay." (Qur'an, 7:12) These ayat clearly show that Iblis was not from amongst the angels - he was a Jinn who was created from fire.