D01 Controller reported aircraft acknowledged traffic in sight then returned on frequency to report a NMAC.

Narrative

I had Aircraft X; VFR; on the practice approach for 12L at BJC at 7;100. As the aircraft was passing BDU pattern; I called traffic on two aircraft at 1 O'clock and 3 miles indicating 8;000. They appeared to be a glider and a tow aircraft; which I included in the traffic call. Aircraft X reported the traffic in sight; and I switched him to BJC Tower. A couple miles later; Aircraft X turned to the east and checked back on with me reporting a near mid-air. I asked who it was with; and they said it was an aircraft doing a rapid spiral descent and they had to turn to avoid. There were no other aircraft in Aircraft X's path; so I believe this was the glider tow aircraft descending back into BDU's pattern. Aircraft X stated they would leave the area and return to ZZZ. This VFR corridor is very congested; and we have multiple approaches that fly through it. Including many jet aircraft. I recommend we either work on adjusting the BJC approach to be farther away from BDU; or do pilot outreach; especially to BDU glider ops; to show them what to watch for; or help work out where it is best for them to do their climbs/descents. Maybe similar to how we work with Parachute Ops at LMO. There was nothing I could have done different in this situation. Traffic was issued; they were indicating 1000 ft. apart; and the pilot had them in sight. The BDU pattern traffic did something dramatic and unexpected. We either start doing pilot outreach; or we start denying practice approaches due to unsafe VFR volume in the area. It is scary to run traffic in this area; and we HAVE TO run all aircraft through this area when we are landing 12L.I will start denying approaches to 12L if I have to from now on. This is getting dangerous.