The screen size of a television is always measured from corner to corner across the diagonal. Therefore you can use basic trigonometry to get the actual height and width of the screen.
For a diagonal of 100" the other dimensions will be: Width 80", Height 60".
So the screen alone will be five feet high and six feet eight inches wide. You can probably add another foot to the height and and about fourteen inches to the width dimensions to account for the frame and loudspeakers.
This will give you a telly of six feet high by eight feet wide. This is an absolutely ludicrous size for a domestic television set. If your friend is telling the truth then the thing would cost well over £20,000 and weigh enough to require a crane or fork-lift truck to move it. The power consumption will be rather high too so it won't be cheap to run.
Also, because of the way that tv pictures are created, it would be unwatchable from any distance closer than around twenty feet. You would see all of the horizontal lines that the picture is made from. It would be like watching the programmes through a row of horizontal metal bars.
Of course, he could be talking about a projector system. This will be much cheaper and more practical than a big conventional tv set.