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. [https://en.wikipedia.org/wiki/Berlin_Ringbahn][1]

Train from Berlin to Bonn can be check here https://www.bahn.com/en/view/index.shtml, get the ticket early fir lower price. At the time I post this, it's 124 EUR.

A month of 4G SIM card costs less than 7 EUR on Amazon. You will get 2 GB data. You can order 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.


  [1]: https://en.wikipedia.org/wiki/Berlin_Ringbahn