When my dad first left my ma started chatting to people online. She then became convinced that one of the people she was chatting with was my dad in disguise... especially when she then arranged to meet him in person and he stood her up.

Then she got with her (ex-)boyfriend and forgot about it all.

However she and Pete split at the start of the week and she's now convinced again. She was going to go visit this guy over the weekend and he's cancelled. So, she's now convinced again that this guy is actually my dad.

She's completely ignoring the fact that my dad doesn't have internet access, that she was talking to this guy online at the same time as I was talking to my dad on the phone... and that once she was talking to him online while I was having dinner with him in the pub.

How on earth am I supposed to convince her?