"My friend is beautiful."
Translation:Η φίλη μου είναι όμορφη.
You are mistaken. The masculine form is accepted. If your sentence was not accepted it means you had another error but it was not the correct use of the masculine form. However, without seeing your sentence we cannot determine what that error was. You should have reported this as: "My sentence should be accepted."
As you can see from the comment above yours the masculine form is accepted. It should be: "ο φίλος μου είναι όμορφος" the adjective needs to agree with the gender of the noun.