The teacher's observation might have some force. Many universities are strapped for cash, and some might be tempted to bung a candidate a lowball offer in the hope that the candidate will take the offer and contribute to the university's shrinking coffers. I hope that I am not being unduly cynical. I could hardly blame a university for trying to get as many people to say yes to its offers as possible, especially candidates who may be well capable of meeting the offer.
Might it not be better for your son to apply for whichever course he is most interested in? If he prefers Law plus French, go for that. If he prefers Law by itself, go for that.