Can a catholic lay person baptise someone in an emergency or if they are in danger of death?
If someone is in danger of death, then yes, it is licit for a layperson to baptize the individual.
Rite of Baptism, No. 16
In imminent danger of death and especially at the moment of death, when no priest or deacon is available, any member of the faithful, indeed anyone with the right intention, may and sometimes must administer baptism.