I’d take a trip to the local septic permitting agency and see if I could get copies of the permit and approved septic system design/as-built site plans. The inspector will probably be able to tell you if you need it or not.
It might be a vent for the leach field although that’s not common where I’m from. I guess it could also be a marker for a distribution box or some other system component.
Bringing problems to their attention becomes expensive, that’s my point. Now you have to deal with the “official” way to fix it, as opposed to the way you can afford.
Sure, that can happen if there actually is a problem. But, I’m failing to see a problem here that could become expensive. This person has a functional septic system and they’re trying to find out what this pipe is and whether it’s important for the function of their system. Getting plans and talking to city/county staff is pretty routine for stuff like this.