I had a buddy in high school and college who had dual French and American citizenship.
He renounced his French citizenship when the Armée de terre came calling, and he couldn’t swing some sort of deal where he could defer his induction and satisfy the obligation by working for a French company overseas for a few years. He was going to be drafted in 1991, and didn’t want any part of it.
Had he been willing to do a two year stint in the French Army, he could have kept both. As it was, it was only really handy for being able to choose the shortest customs lines into the US and France after visiting his grandparents in Paris.
That is interesting because I also came of age at the time that Military Service was compulsory in France, but avoiding it was fairly simple. The rule, if I recall correctly, was that you could serve for France or the country of your other citizenship. Since in the US, service is voluntary, they also had a rule that if you showed that you were in school for the 4 years after you turned 18 you would not need to serve.
There were some rules about, like I could not live in France for more than 3 months during that time period, and if lived somewhere else (other than the US) for more than 3 months I would have to wait until I was 29 to be exempt. Since I was in College in the US for 4 years, it was a matter of getting a letter from my College stating that I had been there for the 4 years in question. There was some additional paperwork, when I first became eligible at 18, but on the whole it was a straightforward process. I am surprised your friend did not know about that option.