Robert Buzalski drank vodka liqueur before setting off for Edzell with Krysztof Birula and another man.
A court heard Buzalski's hire car left the road during bad weather conditions and plunged down an embankment into a river bed.
Mr Birula suffered a broken back in the crash and had to be cut free from the vehicle by firefighters.
Buzalski had denied causing serious injury to Mr Birula by dangerous driving.
A jury at Forfar Sheriff Court found the 36-year-old of Ruthvenfield Road, Perth, guilty of the charge.
The crash took place on 24 August 2013 on a farm road from Cornes Cottage to Dalbog Farm, near Edzell.
Mr Birula was unable to travel to court to give evidence but a statement taken from him recounting the events was read to the court.
The statement read: "Buzalski was driving normally, he did not drive fast.
"Suddenly Marcin Kopij told Buzalski: 'Watch out, we're being drawn off the road'.
"I fell off the seat and I think I hit my head. I said to Buzalski that I could not move an arm or leg.
"The next thing I remember is waking up in hospital."
Giving evidence in his own defence Buzalski, who suffered four broken ribs in the incident, said: "I think about it every day, it's very hard.
"I ask myself every evening, why did it have to happen?"
Sheriff Pino di Emidio deferred sentence until April for social work background reports and released Buzalski on bail.
A man whose car plunged into an Angus gorge leaving his friend paralysed has been convicted of dangerous driving.