Hopefully the demonstration doesn't require you to fly your drone 100 km and back!
To minimize radio interference, look at all the radios in operation. There are at least six radios operating, by my estimate.
Your radios, in order of priority, include the controller TX, your FPV video, and GPS. I put GPS last because you can easily fly an effective mission without GPS assuming it is line of sight (or you really trust your FPV to help you fly the aircraft home). Some flight controllers (e.g., Phantom 4) can use GLONASS GPS signals as well as US, which might improve immunity from radio interference.
Your control radio is top priority because loss of signal could be no fun (especially with no GPS and thus no fail safe). Many RC controller radios use 2.4GHz spread spectrum, so the link can tolerate a few collisions (e.g., interference from strong 2.4GHz WiFi). But DJI Lightbridge (including some Phantoms) may stomp all over the 2.4GHz band, and we are warned not to use other 2.4GHz radios near a Lightbridge. You can get RC TX radios in other frequencies like 5.8GHz (short range) and long range UHF 433MHz (Dragonlink). At a minimum, you should consider using a booster amp and/or directional antenna to make sure your aircraft receives a strong control signal from you in the presence of radio interference. There are relatively easy hacks on the web to add a standard WiFi booster amp (battery powered) to Taranis radios (may require a bit of delicate surgery, depending on your hardware version).
The customer's radios (for the system demonstration) include WLAN and the unknown Long Range Radios (shown in the diagram), and perhaps other voice or digital communication radios used for coordination. You will probably want to ask about the Long Range Radios (frequency, power, is it a tight directional beam, etc.). Definitely stay out of a high-power tight beam radio path, even if it's a different frequency. Also ask whether the WLAN radios used in the demo are standard low-power or some kind of high-power variant. If the WLAN is standard low power and you are not flying near one of the nodes, you should have no trouble from WiFi.
You could avoid all of the radio questions by asking the customer to carefully switch OFF their high power transmitters (other than voice radio) during your filming. For example, you film the deployment of each Helikite with their radios off. Then you keep your aircraft on the ground when they switch on the WLAN and Long Range Radios. Of course your video looks the same whether their WLAN is operating or dead, so this may be feasible.
You can find a list of WLAN channel frequencies here:
https://en.wikipedia.org/wiki/List_of_WLAN_channels
Here are the (US) GPS frequencies:
https://en.wikipedia.org/wiki/GPS_signals#Frequency_information
GLONASS GPS frequencies start at 1.6GHz:
https://en.wikipedia.org/wiki/GLONASS#Signal
