3 Answers

Here is the site your friend needs to access to see if she can become a U.K. citizen > https://www.gov.uk/becoming-a-british-citizen/check-if-you-can-apply

Coming from another European country , she can stay here as long as she wants to. I expect once you are married she could become a British citizen if she wished. I'm  sure coming from Germany she will bring mainly positive contributions to the UK, unlike some of the other people that manage to come a live here just to sponge off the rest of us.

As Germany is in the EC, I don't see any problem with marrying regardless of whether she is a British citizen. If she wants to become one, for example to be able to vote, she can do this later. My dear wife is French and there were no difficulties with marriage. (That is not to say we have no problems in our marriage).