I think it's more straightforward to look at it as Isca's power forcing her to scream out, which distracted Tarn so he didn't see the helmet being moved over his head.
Read the issue again, that doesn't line up.
Isca's outburst was little more than "some commotion in the stands" and Tarn didn't even acknowledge it before accepting Magneto's challenge.
Tarn was stalling for time until his powers came back from what Gabriel did to him, and accepted at the point he could feel them again.
He didn't bother acknowledging the Helmet because its not even close to being a weapon, he was arrogant, overconfident, and assumed he'd be able to dust Magneto with a thought.
That Magneto's helmet might be an anti psychic weapon didn't occur to him. Why would it? Even those of us who have been reading Xmen since the 90s would struggle to think of Magneto ever using it that way.
I think Adder's explanation makes a lot more sense. Isca was EXTREMELY upset at the prospect of Tarn losing to Magneto, called out that she had fought beside him for 1000 years and no one knew him better than she did. If there isn't a romantic connection between the two, there's something damned close. "losing" him isn't something her powers would generally accept.
Tarn is called out as being so hated by everyone that whoever killed him would be a hero to the entire planet. The amount of challengers he's had over a thousand years would be an absolutely staggering number. That "outside interference" was saving him here is absolutely worth considering.
That arena is usually a life-or-death affair, meaning Tarn would be spared from any defeat that would kill him. His sole loss (that we know of) was to Storm who explicitly would not have killed him, so Isca's powers wouldn't have come into play.
Roberto likely wasn't aware of the relationship between the two- he just saw an opportunity to leverage her abilities into a win. Whether the bet explicitly caused the loss or simply short circuited her powers into a no-win situation is probably immaterial. They were definitely involved somehow and the concept that 'yelling' from the stands (which Tarn did not appear to hear or care about) was the deciding factor doesn't make much sense.
