Well she didn't record the act he's accused of committing (attempted rape), she recorded him stating an intention to commit that act. That would be strong evidence in a court but we don't know why she dropped the case, or got back together with him, or if there's any other evidence that could add more context to that recording. Maybe the recording alone is bad enough to fire him from united - I don't know. But I think most people are saying he shouldn't play again because they believe he tried to rape his girlfriend, and I don't think that conclusion should be jumped to without a fair hearing in a court where all evidence can be presented.
And with a lot of other types of crime i would agree with you. But given the nature of coercive and abusive relationships, we know it's basically impossible to have that fair hearing with all the evidence in this case. You're putting a principle over reality in this case. That's fair enough but most will disagree with you.
There's more than enough evidence already public for Man U to be able to make a decision on this. If they keep him then it won't be about his innocence but his ability. If he was a reserve goalie they would have already cancelled his contract.