Canon Law says about sponsors:
Can. 892 Insofar as possible, there is to be a sponsor for the person to be confirmed; the sponsor is to take care that the confirmed person behaves as a true witness of Christ and faithfully fulfills the obligations inherent in this sacrament.
It is your obligated duty to make sure 1) that the confirmand knows the faith and 2) that the confirmand is living the faith. If you knowingly and willingly acknowledge a person worthy in both regards while knowing that either or both are deficient, in my mind you have committed a sacrilege by perjuring yourself before God.
Now, certainly one cannot absolutely know the heart of another, but neither does that mean where lacking in common sense. If you suspect sinful behavior, such as living with a boyfriend or girlfriend, then it is your duty as a sponsor to confront the behavior and make sure the confirmand knows the Catholic moral teachings in that regard. Use tact if needed (or be blunt if you think it needed).
Once more, as this person’s sponsor, you have been placed in a role of spiritual guidance and you must not let any awkwardness, discomfort, or fear of being called names (like a judger :rolleyes:) deter you from safeguarding this person’s soul. You will have failed in your sacred duty if you let this person dance on the road to hell without putting up a fight.
One final thought, if when all is said and done you feel that this person is not living the faith, you should resign from being the sponsor (since declaring the candidate worthy would be a fraud). You would then discuss it with the priest and whoever else should be involved (maybe the RCIA director?). Ultimately, they may decide that the confirmation will go on no matter what, but they can’t force you to participate against what your well-formed conscience tells you to do.