You're not supposed to send the sponsor's passport, so they likely pulled it from the package when it arrived and returned it. And as you used a solicitor, they returned it to him/her.
It means nothing about approval or refusal. They were probably more confused as to why it was included than you are about it being returned.
Have your solicitor return it to you in the US using either FedEx, DHL, or UPS (not Royal Mail or Parcel Force).