I disagree with Bewlay.
A couple of years ago me and my female friend joined our male friend and his mates to watch a big rugby match. There was about 6 or 7 guys - all rugby players so not exactly small and it was obvious we were all together.
My friend had her drink spiked that night. She collapsed on the floor in the middle of the pub, none of us could 'wake her up' while the paramedics were on their way.
Her eyes were rolled back into her head, the paramedic was whacking (really hard!), a metal pen thingy on the back of her hand for any kind of reaction, but there was no reaction at all. They took her to hospital and she was out cold for the next 3/4 hours.
When she woke up, she was paniced and didn't know where the heck she was, I told her what happened and to this day she has no recollection of anything that happened from around an hour prior to her actually collapsing.
Of course, I was petrified as to what was happening to my friend and was going on and on to the paramedics asking what was wrong with her and they said it looked like another case of a drink spike with Rohypnol. I pointed out that it would be senseless for anyone to do this to her as we are clearly no 2 girls alone that can be taken advantage of. The paramedic told me that there was a spate of it happening in my City and it as being done "for a laugh" and that whoever did it was most likely in the crowd of rubber-neckers.
It sounds like your mum is being truthful, dwaty, like I said, my friend has no recollection of an hour or so prior to her collapse.
I hope your dad can believe her and hey can get back on track soon. x