Could an ex-runescape player who has a high level account give you their old high level account

Yes. They just have to give you the password and username.
They can, but it's against the rules of RuneScape. You are supposed to build up your own account. Besides, what's the fun if you already have a high-level account? The fun part is building up your account! But it takes you way too long to build up your own account. So it's better to buy or trade or whatever to get a higher account.