Depending on average transaction size, number of transactions and the potential risks you are taking on (i.e what are you responsible for if your friends product or service isn't delivered on?). Anywhere from 2-20% could work depending on the above.

As for accounting, I would recommend keeping the transactions in a separately coded lines of revenue and expenses so you can clearly show the in and out flow of money for his app. It will be taxed as income so you can include that in your fee if you wish.

Also know that banks these days are very sophisticated with detecting fraud and money laundering so ensure you document the arrangement with your friend so you can easily show everything is on the up and up.

I hope that helps!