A friend of mine is getting ready to quite Galaxy Legion but before he goes he want to give out and share his stuff. He has a few planets that he would like to share the only problem is he occupies them. If he shares the planet and leaves the legion would the alert still be visible for someone to attack the planet?

nope... if he leaves the legion, any planet and npc alerts are cleared
likewise, if you leave the legion, you can't target any alerts you saw while you were in the legion

he would have to abandon, or have someone else that has it scanned take it from him and you take it from them