Is congratulating a friend on the passing of years since she started wearing the niqab, and publicizing that to encourage her and invite others, considered a bid‘ah (innovation) or permissible?

There is no harm in congratulating for pleasant matters such as memorizing the Qur'an, wearing the niqab, and marriage, provided that this day does not become an Eid (a recurring festival). An Eid is what recurs, returns, and people gather for, and is dedicated to specific acts or celebrations. As for merely congratulating without making the day an Eid, there is no harm. For Eid is the name for what returns in terms of customary gatherings, and whatever of it is intended for worship, glorification, or imitation of the people of Jahiliyyah (pre-Islamic ignorance) or disbelievers, it is a forbidden innovation. If you fear that your congratulation will encourage your friend to celebrate that day, or if she already does so, then do not congratulate her. However, if it is in the manner of reminding of the blessing of guidance and encouraging it, without habitual practice or making its time a recurring remembrance and Eid, then there appears to be no harm in it.