"The Ant-Man and Clueless actor, who will appear alongside Fetterman at a Wednesday night fundraiser, initially reached out to the candidate last summer..."
"... just after Fetterman inserted Rudd into a tweet mocking Republican state lawmaker Doug Mastriano for issuing an official letter requesting an audit of the 2020 presidential election. 'In light of recent bizarre, unhinged requests on official letterhead from Pennsylvania electeds, my office formally requested Paul Rudd to re-enact scenes from "I Love You, Man" with me,' wrote Fetterman, who used his own office’s letterhead to issue the request. (While his campaign operations have grown significantly this year, the post that prompted their relationship came at a time when Fetterman, a bona fide Rudd Head, was personally writing all of his tweets.)"
From "THE JOHN FETTERMAN, PAUL RUDD BROMANCE YOU DIDN’T KNOW ABOUT IS HERE/The duo will reenact scenes from I love You, Man at a Wednesday fundraiser for the Senate nominee’s campaign, finally turning the candidate’s year-old wish into a reality. 'Never in a million years did I imagine that it would actually end up happening,' Fetterman tells Vanity Fair" (Vanity Fair).
It's a fundraiser now, but was it, implicitly, part of fundraising at the time the letterhead was used, a time Vanity Fair calls "last summer"*? The tweeting and letter-writing happened in July 2021. Still, Fetterman was a candidate back then too. He'd announced in February 2021.
The idea of using stars to read an old movie script as a fundraiser originated in 2020 — a year before Fetterman's letter — when the Democratic Party of Wisconsin got the cast of "The Princess Bride" to do a virtual table read of the script. This got a lot of press and was roundly praised as delightful and innovative, and, remembering that, you see Fetterman's idea as less weird and wacky and more clearly about fundraising.
I don't think it was the right place to use official letterhead, especially when you want to be critical of other people's misuse of letterhead.
* When people say things like "the things we did last summer" and it's October, don't you think they mean the most recent summer?