M.M.
She is 18 and considered an adult. What you do is give her your reasons for your reservations, tell her you love her and that you will always be there for her. Then when she makes those huge mistakes pick up the pieces.
Let her go to Germany and see this "boyfriend" He may turn out ot be some 50 yo creep. She is an American citizen, she has the protection of the American Embassy should this person want her to do anything illegal, as illegal is considered in the US.
I agree with cancelling any credit cards you have cosigned or have given her.