The former Spice Girl was unable to take part on Saturday night's live show because of an undisclosed illness, but was well enough to make the Sunday night grand finale.
Winner Ben told ITV's Good Morning Britain that Mel must have been very poorly to not appear on Saturday's show, as she'd always been down at rehearsals and working hard with contestants.
"For someone like Mel to not turn up on Saturday… We found out she was ill early on that day," he said.
"She was a very hands on mentor and she's always been there and always been down [at rehearsals]. Obviously it was something pretty serious. I'm sure if she could have got down there, by hook or crook she would have been down there."
The nature of Mel's illness has not been revealed. She only said on Sunday night's show that she felt "better-ish" and thanked the doctors and nurses who had enabled her to return to the stage.
Her husband Stephen Belafonte spoke out today to angrily deny speculation on Twitter that he'd hit his wife, forcing her to miss Saturday night's show.
He wrote: "I don't usually respond to Twitter messages but I will respond to comments of hitting my wife which I think are quite disgusting and untrue! Mel was very ill. A bunch of doctors helped her get better. If fans can't just relax before being negative they are not real fans of Mel."
It's believed Mel spent a few nights in hospital after falling ill earlier this week. She was forced to miss a scheduled X Factor press conference on Thursday and then, the live show on Saturday.
However, contrary to reports, she was in contact with her finalist Andrea Faustini the whole time.
Mel's agent denied a report that Andrea had been forced to fend for himself, insisting Mel had been in touch with him throughout the finale weekend.
She even said on yesterday's show: "I was texting him all evening. He did such a good job - he made the final! I love him."
Mel was replaced by former X Factor judge Tulisa on Saturday night.
Ben Haenow - who was mentored by Simon Cowell - won the finale over bookies' favourite Fleur East, also in Simon's category. He won 57% of the public vote.
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