How are you going to evaluate whether your friend’s son can handle the math for engineering? What criteria are you going to use?
Failing precalc is something to be concerned about, but it is not necessarily a reason for the student to reconsider engineering. There could be many problems. For example, may not have developed the study skills to get (at least) decent grades at a university. Also, how well other students did in this class should be taken into account. If most students are having problems, it could be because the professor has made the class difficult.