Not just any ANT+ dongle - needs to be Garmin's if the software he runs is Garmin. App/kit manufacturers make that connection to protect revenues. You can then pair the dongle with pretty much any ANT+ enabled end device though (foot pod / HRM / cadence sensor /etc.).
Before I got the 910xt I used Wahoo Fitness dongle with my iPhone - worked a treat. Still have it, but keeping it for when Mrs. kfjatek decides to take "this whole triathlon thing" more seriously, so not selling I'm afraid.