Location: Start by completing the Targeted Risk side mission. To unlock this mission, you additionally need to take all of the 13 jumps, whose locations have been marked on the world map in the GTA V - Maps and collectibles guide. It is worth adding that you have, or even should, reach some of the sites on, e.g. a helicopter. You do not need to complete them in 100%, i.e. you do not need to perform ideal maneuvers while airborne, nor land perfectly.
After you take all of the parachute jumps near the dam in the Land Act Reservoir location (to the North-east of Los Santos), there will appear a marker that lets you know about the availability of this mission. After you get there, you should search out the dog shown in the screenshot. Listen to the one-sided "conversation" and reach the dam to meet up with Dom. The freak will try a jump without a parachute on, which will end up in his tragic death.
If you want to complete this mission in 10%, you need to pick up the parachute lying somewhere on the left, after you watch the cutscene featuring Dom. Walk over the gate on the right, hop over the balustrade and use the parachute to prevent sharing Dom's fate. Fortunately, you do not need to land right next to Dom's corpse because this can be, just as well, some more distant spot.
Rewards for the completion of the mission: none
Requirements for the completion of the mission in 100%:
- Leap of Faith - You need to land with the parachute near the spot where Dom jumped.