At one point in the evening, she said she saw the alleged rape victim heading downstairs towards a cinema room, pool and gym and saw Mendy behind her.
She told the court: “She said, ‘I need to go downstairs to get my bag so that I can get changed for work but please can you come and check on me if I’m not back in five minutes?’.
“I said, ‘Yeah, course, why?’
“She didn’t specifically say that she was worried but I could tell by the look on her face she was.
“She just said, ‘Well, Ben’s coming down with me’, and gave me a look.”
The woman alleges Mendy, 28, raped her in the cinema room.
She later told friends she had asked a fellow partygoer to check on her but the woman was stopped from doing so by two men, jurors were told.
Another witness told the court the complainant seemed “fine” after the alleged attack.
The friend, who worked with the alleged victim, said she arrived at her Manchester flat at about 11.30pm after being at the party with Mendy.
She told the court: “She did say she had slept with Mendy. It was a very, very brief conversation.”
She added: “She was fine. I think she’d had a little bit to drink.”
During the party, on July 23, the court was told Mendy’s co-accused, Louis Saha Matturie, “grabbed” a woman while they were in a car after going to buy more alcohol.
The woman, giving evidence from behind a screen, said: “Out of nowhere, he grabbed my neck and pulled me towards his face.”
She said Saha Matturie “gripped” her inner thigh with his other hand and “wasn’t taking no for an answer”.
She told the court she managed to get out of the car when someone came to help take the shopping in but Saha Matturie told her: “Well, later then.”
Mendy appeared to be stopped and asked for an autograph as he arrived at court for the third week of his trial on Tuesday.
He denies eight counts of rape, one count of attempted rape and one count of sexual assault against seven young women.
Saha Matturie, the footballer’s friend and fixer, is alleged to have had the job of finding young women for sex.
The 41-year-old, of Eccles, Salford, denies eight counts of rape and four counts of sexual assault relating to eight young women.
Both men say any sex was consensual.
The trial continues.