Would it be illegal to convince a fellow soldier that he would be better off in the French Foreign Legion?

Technically it's "incitement to desertion". As long as you say nothing about it though, no-one will ever know.

I'm quite sure you wouldn't face any trouble, however, if this service member were to leave to join the FFL before he or she were to ETS, then they would be AWOL and after 30 days be a deserter. Second, if this service member is such a problem for the current service they are in, then they face a wake up call joining the FFL. In my own experience, the military has gone soft in order to cater to all the little momma's boys of today. I guess you could say it's more user friendly.... The FFL is totally different and I have a feeling that your soldier would not survive the first month of training.

For wants of further details, it's hard to answer your question.

"benefit his current military" (?)

The military justice of what country?

Where did you get this idea of suggesting to him to join the French Foreign Legion?! From under your hat?

Sure, i almost joined the french foreign legion, but as you can see, i joined something entirely different.

I think i would not have been better off with the french, but it sure does beat the first country that i almost signed up into.

no. it isn't subject to ucmj. You can even be in the legion before you serve the us military. they can't just switch though. they would have to finish there enlistment.