Mandel replied: "Safe biking my friend."
The joke present comes as Cowell's girlfriend, Lauren Silverman - with whom he has 3-year-old son Eric - has reportedly asked him to ditch his dangerous toys, including the brand new e-bike.
A source said: "Lauren doesn't want him to ride one of those bikes again, so he's agreed with his friends and family to get rid of them as soon as he's out of hospital.
"The power of the bike was unbelievable when he changed the setting up to the second one. The bike just flew into the air.
"Thankfully he let go of the bike as he fell backwards and it didn't land on him."
Mandel recently updated fans on Cowell' condition, when he said the TV star was "doing spectacularly well" after his fall.
He said: "We didn't know what to expect. I knew that we could expect something spectacular because we have, I believe, the most talented crew in the world, in every possible department. We were a little bit in disarray because we were worried. We were worried about our friend, and our boss, and our leader, Simon Cowell, who we hear is doing spectacularly well, considering.
"The latest I've heard is that after a six-hour operation, and some fused discs, and the rod put in his back, yesterday, he was up on his feet, already, he's already mobile! So, as I've been saying to a few other people, I wouldn't count out seeing Simon again before this season's end."