Is it permissible for me to withdraw a large sum of money from my friend's account, which was likely deposited by mistake by the embassy, knowing that he is not entitled to it, and then transfer it to him and take a portion of it for myself? And will I be held accountable for this action?

It is not permissible to cooperate with the aforementioned person in unlawfully devouring people's wealth, and you must stop sending money to him, so as not to partake in sin. The money you sent after knowing that it did not belong to him – you are obligated to guarantee it and return it to the responsible party, after which you may claim it back from your friend. It is not permissible to accept gifts from him, because he does not own the ill-gotten money which is forbidden to take or give. His demanding compensation from the embassy does not make it permissible for him to seize their money without their permission. However, if he informed the embassy of his travel and they left him what you paid him, then there is no harm in transferring it to him or taking what he instructed you to take, and you are absolved of the guarantee for what you previously transferred to him.