I agree that Meg's father is a bit of an oddball and its hard to trust him. But I'd say he was less likely to tape a conversation than to release a letter..
the best thing IMo would have been for her to visit him before she got engaged.. not left things to just before the wedding which is when he seems to have become a bit flakey, and then ended up not able to go...
I don't know with the influence of Samantha and the media I have no doubt that a conversation would have been recorded. Also if it was only recorded one way then he could have lied about pieces from the conversation. He has shown that he has no problem lying about his daughter for money.
That being said, I have frequently seen it recommended to someone in a difficult relationship to write down their thoughts they want to say/share with the other person as a way to release the feelings.
Based on what happened I believe that a letter was actually the best action.