My DS was the same at 18. Got a job and then started making plans to move out, renting a flat with a couple of friends.
Every time he mentioned it, I'd drop in another little thing that they would have to pay/do ie deposit, bills, council tax, food, cooking, washing, cleaning, how I wouldn't be around to borrow the odd £10 from etc
They all, not so slowly, realised that maybe they would be better off at home for another wee while 
DS is now almost 21 and still here! I never told him not to go, in fact I always made it clear that he was welcome back anytime, but I did let him realise himself that maybe it was better to wait a while.
I think if your friend tries to get her DS to change his mind he will just dig his heels in and be more determined to move out.
When the time is right and he can afford it my DS will move out, it's just not right now thankfully