A friend of mine has been working for a company for 4months now, its ft but hes not got a proper job description or anything. If he wanted to leave how much notice does he have to give?

None.

According to the guys I spoke to at ACAS about my old job, if you dont have a contract, and have worked less than a year you can leave straight away with no notice period.

After a year its considered prudent to give a weeks notice for every year you work. However its not enforcible unless part of a contract.

Careful however, verbal contracts contrary to popular belief, do actually count for something, if there is an understanding that he will give them reasonable notice, then he needs to do just that.

As pointed out by M3ta7h3ad, tell him to follow the standard system of 1 week, to cover himself at least.

and enjoy a fight getting paid wages :oD

He should mention this to his employer TBH & maybe Inland Revenue... He could be getting paid cash in hand without realising it.