Hmm, you know, that is a typical dilemma about the relationship with friends. From the one side, you do your work as a professional, you have a lot of experience in this sphere and, as a result, you exactly have a right to earn reasonable money for that. But from the other side, the man who is asking you to do that is your colleague and if even he said that he would pay the whole sum, he expects for a little discount. So, a cheap proofreading (like this:https://www.onlineproofreadingservices.net/our-cheap-proofreading-services/ ) is about 11-20 pounds sterling per hour. But it could differ a little bit.