mum2taylor sorry to hear this.
I would suggest first port of call is to ring his line manager and see if he/she is aware of the letter. It may be a miscommunication between management and HR or something, so get him to ask his manager about it. If his manager doesn't know about it or doesn't agree with it, he/she should be able to speak to HR on your DP's behalf.
It could be, for example, that a disciplinary letter is 'triggered' by non contact and no one has told HR that actually there has been contact, or something like that. So it could all get cleared up very quickly.
If his boss is aware and supports this action, and can't get it dropped, DP will need to attend a disciplinary. Is he physically able to do this at the moment? He could request a postponement.
When he goes, he should take a colleague or union rep with him - he should have been made aware of his right to do this. The claim is that he didn't make contact and didn't provide evidence. Clearly he did both of those things, so he should be fine, and no action in terms of a warning or anything should be taken. I expect he has the texts in his 'sent' file, and does he have a copy of the medical certificate. He should also check that his manager will back him up that both of these things have been received?
So it seems horrible, and it is, but actually it should be ok. My best guess would be that it's a bit of a cock-up somewhere, which is why he should try his line manager first to get it ironed out. But if not, and if he has to go, he has excellent evidence that what they are saying is rubbish, so he should be ok. Also, companies are (and should be) usually extra careful with employees who are off sick because of disability discrimination. It is perfectly possible your DP could consider himself disabled, even if temporarily, so they should be careful how they handle a situation like this. I wouldn't suggest he goes in talking about discrimination at this point, but it's just another factor that puts him in a strong position.
Do post again if you need to.