"I can't do that. It's my dad's favourite singer, he'd kill me," he pleads. Cassidy politely tells Mathis that his friend is sleeping.
This, however, is not a tale of Best thinking himself too important to speak to a man who has sold over 350 million records worldwide. Rather, he had a game for Northern Ireland the next day and deemed that his priority.
Some 40 years on, Cassidy talks just as passionately of Best the person as he does of the player.
"My Newcastle boss, Joe Harvey, once said to me, 'Wow, he is the best I have ever seen'. I said to Joe, 'You're right, but he's an even better man'. That tells you what I thought of him," says Cassidy, a Belfast boy like Best.
Now 65, he recalls: "After we played Holland in 1976, Johan Cruyff and Johan Neeskens were chasing Bestie, fighting over his shirt, but George had promised it to a little boy with meningitis in Belfast and told these two great players he wasn't going to let the boy down.
"We were at the cinema in Belfast on another occasion and George went to the front of the queue and paid everyone in. He wasn't showing off, he just earned more money and wanted to help people out."
It was May 1971 when Cassidy, then 20, met Best for the first time before his debut, a 1-0 defeat by England at Windsor Park. It was a match memorable for Best's disallowed goal when he flicked the ball away from Gordon Banks and headed home as the goalkeeper tried to kick clear.
"You felt in the presence of genius. Here were these great England players like Bobby Moore arguing over how to stop him," says Cassidy, who went on to spend a decade at Newcastle, the city he still calls home.
"We were in the showers afterwards and George said, 'Fancy going out tonight?' My mates couldn't believe it.
"He bought me my first alcoholic drink at the Abercorn in Belfast. We sat at the back for a cabaret show but when the lights went on everyone saw Bestie and that was it, the women flocked towards us.
"I was a kid, still a virgin, I didn't have a clue. I rang home and my mum said, 'Get yourself back to the hotel now'."
Within a couple of years Best had picked Cassidy as his room-mate. "He liked me as a player and probably didn't think I was a threat looks-wise," he smiles. Cassidy's mum, Isabell, was concerned. His dad, Tommy, was proud.
"Mum wasn't sure about the womanising and me being led astray," he says, "But one day I was heading home and George said, 'I'll come with you'. My dad didn't know what to say, George was his hero. Even my mum was starstruck, and she was the one warning me to be careful around him! He was brilliant with them. He was a working-class lad and just had this charm.
"After six months I realised how warm a person he was. He was the opposite of a big-headed superstar."
There were occasions, however, when Cassidy was reminded of just how big a star Best was.
"We were playing Portugal and by the time we landed he had every air hostess's number," recalls Cassidy.
"I was rooming with him and three of them turned up, all at separate times. Miss World also called by once. That's when I realised how attractive he was to women.
"Everywhere we went people liked him. We arrived in Moscow to play USSR in 1971 and decided to take a walk. We didn't have a clue but the KGB followed us. They were frightened someone might do something silly to George - now that would have started a war!
"Everyone recognised him but he would stop to say hello. When we got back to the hotel a KGB officer shook George's hand and thanked him for being so respectful."
One of the last times they saw each other was in the mid-Eighties when Cassidy, then manager of APOEL Nicosia in Cyprus, invited Best and Kevin Keegan to play in a one-off match.
"That was some experience," says Cassidy, who wants to return to the dugout in the North-East non-League scene, 'but off the pitch he wasn't the same lad. He admitted to me then he was an alcoholic.
"It's an illness. He had enormous pressure - he was one of the most famous people in the world and couldn't handle it in the end.
"When he died the funeral was on a Saturday. I was manager of Workington so I rang his dad, Dickie, and explained I had a game but wanted to come.
"He said, 'Tommy, what would George do? Go to the match, son'.
"My wife, Rosemary, and I were driving to the game, listening to the funeral on the radio. The Irish singer, Brian Kennedy, sang Raise Me Up. I pulled over and burst out crying. He meant so much to me."
Ten years on, Best has not been forgotten.
