Samsung Galaxy 6 sets off Apple Pay on iPhone 6
Apple Pay works with NFC "Near Field Communication" band at 13.56 Mhz (but in low energy) means it has to be very close to the source to work).

You iPhone gets a interference signal (not intended) from Samsung and it thinks it is near a payment machine and it activates.

Since it is a interference, there is no software that can fix that, it is a hardware issue (shielding).
(BT and WiFi) or keep them apart.

So your choices are, turn off the Apple Pay, turn off the Samsung payment feature, or keep them apart.

Edit: upon closer exam of the "Samsung Galaxy s6" model it shows to have the NFC capability, that would also explain the interference.  (and you may be able to solve the problem by turning the Samsung's payment feature off.)