If the request for ex parte reexamination is filed by a third party and not the patent owner, the third party may not participate in the ex parte proceedings beyond the filing of a reply to the patent owner’s statement under 37 CFR § 1.530, if the patent owner files a statement. No other reply or submission by a third party will be considered in ex parte reexamination.
— see MPEP 2200, and inter partes reexamination for more