The friend said she was with the alleged victim on the night the girl is said to have performed a sex act on the 28-year-old.
She said her friend was 'excited' and 'in a good mood' when she returned from the meeting.
The girl's friend told police she did not believe her at first, "because he's a footballer, he's famous and she's only 15".
But she said she changed her mind after a seeing a message that Johnson had sent to her friend which said: "Just wanted to get your pants off".
In her police interview, she told an officer: "That's why I know something had happened between them for definite."
The friend also gave details of the night the girl revealed she and Johnson had engaged in sexual activity.
She claimed the schoolgirl 'got a bit drunk' and boasted about what had happened.
After Johnson failed to meet the girl for a third time, she had posted a Snapchat photo from outside his house, the court heard.
The jury were told the girl's friend had spoken to police again last week and told them of more serious sexual activity which her friend had alleged, but about which she had not told officers in her earlier interview.
The girl said: "She [her friend] told me not to say anything."
She added: "She told me that she'd been texting him before she went away and he offered to meet her for money."
Johnson's girlfriend, Stacey Flounders, who is standing by him despite the alleged offences having happened when she was pregnant, was not with him as he entered court today.
Johnson, of Castle Eden, Co Durham, has admitted one count of sexual activity with a child relating to kissing the girl in his car on January 30 last year.
The former Sunderland star has also admitted grooming the teenager but denies two further charges of sexual activity with a child, described by prosecutors as "more serious".
The trial continues.
