You can buy monthly train ticket for around 70 EUR (2 rings), much cheaper than weekly ticket but it depends on what route you will go on. https://en.wikipedia.org/wiki/Berlin_Ringbahn
Train from Berlin to Bonn can be checked here https://www.bahn.com/en/view/index.shtml, get the ticket early for lower price. At the time I post this, it's 124 EUR.
A monthly 4G SIM card costs less than 7 EUR on Amazon. You will get 2 GB of data. You can order it yourself and receive it at your friend's home.
Aside from Turkish and Asian foods, McDonalds and KFC are also reasonable, from 5 to 10 EUR per meal.
I usually have short trip to Hamburg for business and I can say 1000 EUR is more than enough for a month.
