In a cheeky post, the 30-year-old captioned: 'It really is a small world, was walking down a random street in LA & bumped into this guy #nopunintended had a great laugh!'
Guy then commented, jokingly, 'Nice shirt noob'.
Johnny made the shock announcement in August last year that he had been diagnosed with brain cancer and had to undergo emergency surgery to remove a 7cm tumour, followed by chemotherapy treatment.
A year on from his surgery, the actor posted to Instagram, 'Today is exactly a year since i was rushed to hospital to have an emergency operation to remove a brain tumour, it has been a crazy ride and I'm not quite finished yet but i want to share some snaps of the past year and say thank you to everyone who has been there for me and supported me throughout this time.'
The pair have a long history together, with Guy taking on the role of Johnny's mentor during his stint on The X Factor in 2011 before becoming a pillar of support during his cancer treatment.
On the red carpet at the Aria Awards last year, Guy gave fans an update on how his mate was doing.
'I speak to Johnny a lot so I saw him literally not long ago and look, he’s such a great guy,' he told Yahoo 7 . 'But he’s got a lot of good people around him I think and I’m there for him. I’ll always champion him and I have from the start.'