I can understand kicking up a fuss, if you felt it was unjust and you'd worked there for a year or more. But still in probation period its not worth the stress/hassle of it.
If Allen is relying on this job as a reference, then leaving peacefully is the best thing to do in my opinion. Much better likelyhood of a good reference, "left of own accord" rather than went through disciplinary and was sacked, which could happen if he decides to challenge it.
Its allens choice at the end of the day, we can only offer advice. He's the one that has to live with the concequences of his actions.
TiG