At this point hindi pa kayo pwede makasal since the foreign divorce decree still has to be recognised by the local court (not local civil registrar). Sa Regional Trial Court ito fina-file and best for your partner to have a lawyer in the PH that he can liaise with.
Readings:
https://lawyerphilippines.org/2017/04/11/recognition-of-foreign-divorce-in-the-philippines-process/
https://www.linkedin.com/pulse/guidelines-filing-petition-recognition-foreign-joyce-domingo-dapat/
https://tokyo.philembassy.net/consular-section/services/civil-registration/judicial-recognition-of-foreign-divorce/
Basically current pa naman yung process details with the exception dun sa shinare ni @cucci na it doesn't matter who the petitioner was in the divorce and irrelevant na rin if the petitioner was still Filipino at the time of the divorce and siya yung nag-initiate ng divorce.
Although while petitioner in mentioned case submitted her petition sa civil registry, 'yung local court pa rin 'yung dapat mag-issue ng decree na finally recognised na yung divorce between the two people. If you read diyan sa G.R. No. 221029 sa huling part the case has been returned sa court of origin for further proceedings and reception of evidence as to the relevant Japanese law on divorce (since Japan ang nag-grant ng divorce in her case).
The court still has to review na lawful sa country kung saan na grant ang divorce before PH can officially recognise it... ayun lang.
Sa ngayon it's not as easy as correcting mistakes sa civil registry record because in general PH still does not recognise divorce.
Just to clarify lang your partner is also a former Filipino?