To be clear, absent an explicit policy requiring employees to report back to the job for the balance of the day if they are dismissed from jury duty early, the employer cannot lawfully terminate an employee for responding to a jury subpoena, going to the courthouse, being dismissed early and not reporting back to work the same day.
While courts throughout the country have had a “mixed” history, at best, of endorsing other exceptions to the employment at will doctrine, the courts have almost uniformly endorsed wrongful termination claims based on reprisal for participating in jury service. Notably, it appears that Pennsylvania was one of the earliest jurisdictions to prohibit reprisal against workers for jury service. See Reuther v. Fowler & Williams, Inc., 255 Pa. Super. 28, 386 A.2d 119 (Pa. 1978).
If the employer’s “defense” is she could have returned to the job but it did not have an unambiguous policy already in place requiring employees to return for the balance of the day, then it will in all likelihood lose in court. To this point, the OP stated “the policy doesn't say anything” concerning returning to work that day following dismissal from jury duty. I do not think a PA court will give the employer a pass by merely contending she did not make her way back to work that day.
In addition, the unusual offer to pay for two months of COBRA premium coverage definitely suggests that the employer realizes it has probably made a big mistake in firing her under these circumstances.
In sum, without a prominent “return to work after abbreviated jury duty service” policy, this employer is likely toast.