As an acro pilot, my advice is determine whether it violates FAR 91.303. If it does, report it. If not, and you have a way to contact the pilot, do so and express your concern privately.
Most regular acro pilots are quite attentive to FAR limits, because they know that our sport depends on continued support and tolerance by the FAA. If you have ever been to a contest or training session, you will find they are quite strict about safety rules, ESPECIALLY airspace and traffic deconfliction. Every acro pilot's second worst nightmare is being in an inverted 45 down line and seeing an airplane in your windscreen.
Individual yahoos doing casual acro are sometimes less responsible. I suspect that is who you are dealing with here. If so, and they are indeed violating the FAR, reporting them does the rest of us a favor.
There is no regulation prohibiting aerobatics near an airfield, traffic pattern, or approach. It may not be smart, but it's legal in the absence of other violations. The most relevant reg is the restriction on acro below 1500 AGL. That should deconflict with the pattern. What aircraft was the other aircraft at?
Airfields sometimes do have box waivers allowing acro below 1500'. Those boxes will be NOTAMed and should have some sort of comms or observer requirement for compliance.
If the pilot is performing acro inside B airspace, the FAA probably already knows, though they may not be able to identify the aircraft if it is operating without a transponder and ADS-B. If that were a recurring event, I imagine it would become an LEO investigation.
Acro UNDER Bravo airspace is not restricted, except of course where controlled airspace extends to the ground. I fly acro under the Tampa class B shelf with 6K floor, and have spoken to the FAA about it. Perfectly legal and safe.
It can be really hard to find a spot near a class B that is more than 4 miles from an airway. I and others have spent hours scrutinizing sectionals to ensure compliance with that rule. TBH not super related to safety these days. I rarely see traffic on the two airways that pass within sight of our airfield.
That leaves acro over a congested area. Note that is ground congestion, not aerial. I am not well versed on the FAA definition of congestion, because I think it is insane to fly acro over an area without plenty of options for an emergency landing or bailout.
If you are willing to post a map snapshot, I am happy to take a look and give you my opinion.