While it is technically possible to have the Microsoft SAPI speech recognition system transcribe freeform speech (I.E. not predefined like normal commands), E.G. using VoiceAttack's dictation mode, getting accurate results requires as close to ideal circumstances as possible, including a quiet environment, microphone well-suited to speech recognition, but also clear and consistent pronunciation and enunciation.
They can certainly try speech recognition training, and see whether the resulting accuracy after many training sessions (three is considered the absolute minimum) is anywhere close to what would be practical.
Do also keep in mind that any spoken words must be known to the speech recognition system.
The Microsoft SAPI speech recognition system comes with a default dictionary for the language the engine is intended for, but that can be expanded by manually adding words (and ideally pronunciations) to it.
To add to the dictionary, click the wrench icon on VoiceAttack's main window, and on the "Recognition" tab of the VoiceAttack options window click the "Utilities >" button, then choose "Add/Remove Dictionary Words"
As to text-to-speech, Windows requires applications to have focus to receive normal keyboard input, I.E. you'd need to bring the Discord window to the foreground to be able to type into it.
However, VoiceAttack can perform text-to-speech using SAPI-compliant (or Speech Platform 11) text-to-speech engines.
Combining that with something like a virtual audio cable, or VoiceMeeter (which is unrelated to VoiceAttack), would allow VoiceAttack to directly "speak" into Discord by having audio from a playback device sent to a recording device.