If an instituted lector was present, then she should not have done the reading.
From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“99. The lector is instituted to proclaim the readings from Sacred Scripture, with the exception of the Gospel. …
101. In the absence of an instituted lector, other laypersons may be commissioned to proclaim the readings from Sacred Scripture.”
It is debateable whether someone not baptised is included in this “other laypersons” category. The parish priest decides who does the reading, if an instituted lector is not present (see GIRM 107).
Generally, I think, someone not baptised should not do a reading. I base this on the description of a Celebration of the Word of God in the RCIA book. “87 Readings: One or more readings from Scripture, chosen for their relevance to the formation of the catechumens, are proclaimed by a baptised member of the community.” (see the RCIA introductions for England and Wales are at
catholic-ew.org.uk/liturgy/Resources/Rites/RiteRitual.html#RCIA .)
If the person was already baptised as a Catholic, but just being confirmed, I would not object to their selection.
If the person was becoming a Catholic at that Mass, then I think the bishop’s permission is needed for them to read. From the Directory on Ecumenism at
vatican.va/roman_curia/pontifical_councils/chrstuni/documents/rc_pc_chrstuni_doc_25031993_principles-and-norms-on-ecumenism_en.html :
“133. The reading of Scripture during a Eucharistic celebration in the Catholic Church is to be done by members of that Church. On exceptional occasions and for a just cause, the Bishop of the diocese may permit a member of another Church or ecclesial Community to take on the task of reader.”