Date: 202105
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Aircraft: Light Transport; Low Wing; 2 Turbojet Eng
Flight plan: IFR
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Second aircraft: Small Transport; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Enroute; Handoff / Assist
Reporter qualification: Air Traffic Control Fully Certified
Anomaly: ATC Issue All Types; Conflict NMAC; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: When I came into the area; the controller working [Sector X] asked me if I would sit down on the D-side. It was obvious the controller was feeling busy. I asked him if he would like the sector split and he declined. I sat down on the D-side and attempted to get a briefing but the controller was too busy.During the stint; the controllers work was sloppy; he did not seem to have any idea what was happening in the sector at any given time. Several times throughout the stint he did not use positive separation between IFR aircraft and had what I would describe as 'separation by luck.'The reason I decided to report this stint is because the controller nearly caused a midair collision between Aircraft Y that had departed ZZZ1 and Aircraft X that was descending for ZZZ1.When Aircraft X called for a clearance airborne off of ZZZ; the controller did not issue him and IFR clearance but instead instructed the pilot to remain VFR and take up a westerly heading for active military airspace. The only problem with this plan is that the airspace didn't start until 17;000 [ft.]. The aircraft should have been given the clearance to 170 and a heading to get out from under the airspace.Aircraft X was on a heading (for the military airspace) and was descending quickly because he had been left high.I made the following suggestion to the controller. Clear Aircraft Y on a 190 heading and leave Aircraft X on his present heading until they pass.The controller instead cleared Aircraft X direct ZZZ; putting him in conflict with the climbing Aircraft Y that had not yet received a clearance. I just don't understand why he would do this. Direct the airport wasn't even a good way to set the airplane up to land had there not been traffic.When he finally saw the conflict (did not respond when I pointed it out to him). He froze. The control instruction he gave was 'UHHH Aircraft Y maintain VFR' He then called the traffic to Aircraft Y which was at 12'oclock and 10 miles (estimated) opposite direction. Descending.He then followed up the instruction to Aircraft Y to maintain VFR at 8;500. At the time he issued the instruction; Aircraft Y's indicated altitude was 082 and he had been climbing in excess of 3000 ft. per min. The pilot said he could not stop at 085 but could go back down. The next radar hit was at 094. The controller then said 'UHH just maintain VFR.' He should have left Aircraft Y climbing and turned him. It would have cleared the conflict quicker.He then turned Aircraft X 15degrees right for traffic. But when he issued the instruction; he didn't say it with any urgency at all.I don't remember if he eventually issued Aircraft Y a turn or not; but I do remember thinking I was about to witness a midair collision because a bad controller was sitting on the sector.The traffic was not busy; it had some complexity due to the military airspace; but it wasn't bad. It could easily be worked by a competent controller without a D-side.In summary: Bad control judgement; Bad control technique; lack of control instructions; inadequate control instructions; failure to scan the radar environment; and a controller being completely overwhelmed by traffic nearly lead to a collision at near 10;000 ft.




Synopsis: Center D-Side Controller reported a near midair; which reporter advised was due to a Radar Controller's control technique and reported inadequate control instructions.""