OldPilot posted text from Appareo's website, manufacturer of the Stratus. All ADSB devices speak the same language, called "GDL-90". Consuming devices don't talk to the ADSB receiver, they simply listen. All the data - metars, traffic, tfrs, nexrad, etc - are passively mined from a data stream coming from the device.
In our testing, we found our product worked great with the original Stratus 1; the data stream looked like any other ADSB device. But since we don't have an agreement with Stratus we always claimed it worked, but was "unsuported". With the introduction of the Stratus 2, we found the GDL-90 data stream was obfuscated, so we were unable to support that product.
I'm just presuming, but it seems Foreflight is using the Stratus as a technology hook, so owners are unable to choose another navigation package without abandoning their Stratus investment. It would not suprise me if Appareo "upgraded" the Stratus 1's firmware so it would be rendered unusable with other navigation apps. You might check with the prior owner, see if they ran a firmware update on that device.
-Walter