I discussed this with a couple of my tech reps.
Pulsar:
For clip-ons, my suggestion would be our
Pulsar Krypton 2 FXQ50 Thermal Imaging Monocular with its 640x480 <40mK sensor and 12µm pixel pitch. It is capable of up to 2,500 yards of detection range and can be used as a monocular when paired with a Pulsar 3x20B quick-release monocular attachment. The kit version, which is what you'll mainly find available, comes with pretty much all the adapters for use with daytime riflescopes. The monocular attachment is sold separately.
I will say, dedicated thermal riflescopes are much more popular than clip-ons, but the Krypton 2 XG50 is still popular in regions where restrictions don't allow for a dedicated thermal riflescope or for guys that already have a dedicated daytime riflescope setup that they are set on not messing with. They are fairly popular with LE/Gov customers as well.
AGM:
The ability for him to be successful with a cli-on being mounted on his duty weapon will depend on the daytime optic he is using and the distances he will need to engage. I know the majority of the PD engagements even with their precision rifle would be under 100 yards, but a lot of these guys run daytime scope with a high base magnification which can make clip-on units difficult.
I would suggest that he spends the extra money for the 640 sensor on this purchase, there are two units listed below that I think would be the best for his situation.
AGM Secutor LRF-C 50-640
AGM Rattler-C V2 50-640
Both will have optional eyepieces to use as a monocular
