With a little help from a big friend you will see in the video below a Nassau grouper eating a small healthy lionfish. Sorry about the sound – you may want to mute that. However, if you want to hear the reaction of the diver keep it on a low volume. The actual eating part takes place near the end of the video, at 2:20 into the video.
Note how the Grouper struggles to swallow the lionfish. When Groupers eat dead lionfish speared by a diver they will approach the lionfish head on. That way the spines lay down when the lionfish is swallowed. However, eating a live lionfish is a much trickier process. The lionfish will turns its head away from a potential predator and splay out all its 18 venomous spines. If it is grabbed or ends up inside a predators mouth those spines can do a lot damage and they pack a venomous punch.
If you want to see what other creatures eat lionfish have a look at my post What eats lionfish? It has a lot of other interesting videos I think you will like. Other interesting posts on my site that I would recommend are Lionfish spines, What do lionfish eat? and Do sharks eat lionfish? They do now!