Ever since her last childbirth whenever my mom (60yo) is ill, her temperature, instead of rising, plummets. can anything be done to help this problem?

Unusual: The temperature is an indicator, not a problem of its own. It is possible for someone who is very ill to have a lower temperature, but this is usually a grave sign. Sometimes when people are sick, and especially if the mouth is dry, an oral temperature will indicate a low number but the core body temperature is actually higher. A rectal temp below 96 deg f should be evaluated immediately.