Is this possible at all
Very definitely, yes.
he's on 124Khz
Are you sure about that frequency? 124kHz is not a frequency used by any two way radio equipment I know of? That is a frequency for worldwide communication, the wavelength is measured in kilometres!
The interference could be coming from a number of different sources. As indicated above, it could be the Power Supply (when under load), the Mikrotik board or the wireless cards. You have to remove or swap out (with something completely different) to test. So, to test the PSU, as suggested work from just a charged battery only. No mains, no solar panels, no electronics controlling this. Just a normal battery directly into the Routerboard.
Then, consider removing the wireless cards and pigtails - just leaving the Routerboard only.
Then change the routerboard for a completely different model. (We found for example that all our RB532A's used to wipe out a VHF service across the whole island, we tried everything, checking earthing, adding ferrite blocks, but the VHF signal was being passed up through from the Routerboard, through into the R52 cards and then into the 2.4GHz aerial via the coax on the mast! In the end, changing to RB433's solved the problem even though we used the same wireless cards, outdoor boxes, coax, aerial etc. All we changed was the Routerboard.)
If the interference heard on his radios is a hiss sound - this suggests a switched mode power supply. If it were more pulsed and harsh a sound, I would suspect the 5.8GHz RF side. You can prove this by changing the RF mode of the wireless card between 802.11 and nv2 as they will sound different. Also you can disable the wireless cards in winbox and see if the interference sound changes. If it is the RF the interference should stop and start as you enable/disable the card.
Ron