thank you for sharing your situation with us in the community.
As for your question:
Your fiance in Poland has to get in touch with the Civil Registry Office. You have to submit a number of documents: a proof of identity (e.g. a valid passport), a certificate of birth (translated into Polish), a certificate of no impediment and (if necessary) certificates that prove that previous marriages have been divorced etc. The Registry Office will guide you through the procedure from there on.
For extensive information about marriage in Poland please check out this website:
I hope I could help you both, if you need further guidance please do not hesitate to use our community.