Can we just knock this 'employer can't give a bad reference' thing on the head please. So long as its fair, the employer can give whatever sort of reference they wish. Many employers may decide not to give a bad reference but its perfectly legitimate to do so so long as they have evidence to back it up.
A poor attendance record can certainly be stated on a reference. If there's a good reason for it though, such as a broken leg, it probably wont go against the person.
If your friend is still within the first 12 months of his contract it's going to be difficult to form a case unless there's discrimination involved, which doesnt sound likely from what you say here.