I would suggest to let both owners simply work it out between themselves. There isn’t much the OC can do without concrete evidence of a breach of by law. The OC most probably wouldn’t stand a chance at the tribunal; so far it sounds like 1 owners word against another (hearsay). Even if the OC issued a section 45 breach notice, taking the matter further may be pointless without some evidence of excessive noise. You should direct both owners to take the matter through mediation and adjudication if necessary, it will then be on they’re onus to prove these allegations to the adjudicator or tribunal.