So how did he know this random post liker was coming and he could barely see her at the back of the room, and didn't know what she looked like
He didn't know. 'Mediums' wouldn't just prepare for a handful of people, for that very reason, he'd prepare for lots and see which one gets a hit. He didn't need to see her, or know what she looked like, because you said he asked if there was anyone with the surname XXXX - and she put her hand up, volunteering herself. And at that point, she'd bought in to it wholeheartedly.
There's elements of preparation, research and cold/warm reading etc all employed at the same time. You say he got everything right - that's because she told you he did. You admitted you don't the details yourself, so you're relying on her tellings.
As an example: Let's say he reeled off four names. The first three are correct (could be down to research or could be Barnum effect. The last one he says is Edward - except it isn't Edward it's actually Edmund, but by that time she is so invested, she gives him a bye - 'oh he might have misspoken' or 'oh he was usually known as Ed so easy mistake' etc. She tells you he got all the names right, straight off - because that's what she's told herself.
The same is true for the 'specific' facts. Unless you personally know the full detail of said specific facts, she is an unreliable witness. Only two of the specific facts out of four may have been largely correct. She'll forget the two that weren't when recanting the story.
There's also the very real likelihood that the names and 'facts' would have been true for a number of people in the audience. Your age, demographic etc can all be used to come up with a picture that is likely to have some truth in it. A seasoned medium will be all over that.
One last question, I'm not sure how old you are but do you remember Paul Daniels? Do you believe Paul Daniels made an elephant disappear on live television (there were also independent witnesses present with him)? If not, why not?