Why is に used in that sentence?


> でもほんとに、エピソード21 **に** は涙が出るほど笑ったよ。

For some reason I feel like it would be better without the に (in bold) but I
can't explain why.

Another person said that で would be better but I think that it changes the
meaning completely. I mean that （登場人物の）Aさんは エピソード21では涙が出るほど笑ったよ would be okay
but if the subject is not a character of the episode で is not appropriate.

Could you explain the reason behind using に here?

