My boyfriend has episodes that sound something like that, but I doubt they're seizures. They don't interfere with his life in any way, so it doesn't really matter I guess. (your fiance should probably get checked out though anyway if his episodes are bothering him.) sometimes in his sleep, my boyfriend becomes concious but can't see, move, or speak. I can tell when he's getting it because he tends to chatter his teeth, jerk, and/or moan. I have to shake him really hard to get him to snap out of it and wake up. It doesn't happen all that often, but when it does it usually happens a few times. It seems to only happen right after he has fallen asleep.
Also, one time my roommate mentioned an experience like this to me. I had come in the door and he was asleep on the couch. He said he was aware of me and could see me, but he couldn't move or speak. He wanted to call my name and tell me to come wake him up but he couldn't.
Maybe this is just one of those things (like hiccups) that just happens but is no big deal. Some people get it and some don't. Who knows. But like I said, your fiance should still get checked out.