Hi Leroy,
1)When your friend is in the UK you will dial internationally using 0049 1…
2)To place calls her phone must be used just like yours; in other words she would dial 07…
3)None; when you will call her you will pay international charges as if you were calling Germany (which you will because the call will be redirected from Germany back to the UK); your friend will also pay roaming charges for receiving calls; when she calls you she will pay a rate that is usually higher than what you pay for local calls (rate info will be provided to her by the German wireless carrier).
I hope this helps. Perhaps you would be both better off buying a UK prepaid sim card for her phone.