A couple things: First, my understanding is that a lay person may only baptize if the person is in imminent danger of death. I do not think being in a stable coma would count, since a priest or deacon could do so later.
Second, I’m pretty sure in order to baptize an adult who is incapable of expressing her desire to be baptized, there would have to be a reasonably certainty that she would have desired it. (For instance, if he was taking RCIA classes). I do not think baptizing incapacitated people who have never expressed interest in it would be considered a good idea.
But if you’re concerned about her, the best bet is just to talk to a priest. Express your concern, and ask what can be done.
EDIT: I am in no way an expert on this sort of thing, but the wording of the following canon seems to support what I just said:
“Can. 865 §2. An adult in danger of death can be baptized if, having some knowledge of the principal truths of the faith, the person has manifested in any way at all the intention to receive baptism and promises to observe the commandments of the Christian religion.”
(Which seems to imply that if such a thing is not present, then that the baptism ought not be done.)