Although I do believe she would, what is the actual evidence that she cheated?
There's no hard evidence, of course, mostly just timeline stuff and him showing up in vlogs and things.
Timeline is basically after what, like 7 years? with Joshua, they get married in July 2015. 'Haters Back Off' when Colleen first meets Erik starts filming in April 2016. In June 2016 Erik can be seen getting pretty cozy with Colleen (sans wedding ring) in one of the Ballinger Family vlogs of Parkers 3rd birthday. September 2016 Colleen and Joshua announce their divorce (wherein Joshua makes it clear this was not his idea and not what he wants)
Then in November 2016,what everyone refers to usually when talking about their suspected cheating, Erik can be seen in the background of one of Rachels vlogs from their family trip to Hawaii. Then again in November 2016, Erik is invited along on their trip to Vegas. December 2016 Erik is spotted again in the background of one of Rachels vlogs from their trip to the mountains.
He was seen in the background of vlogs and things through 2017, but I'm just focusing on 2016 since that's when they met and when she and Joshua split.
As far as their public claims, her and Erik started dating in early of 2018. Based on Flynns birthday she had to have gotten pregnant sometime in March of 2018, they got engaged in June of 2018, and then married in Nov 2018.
It's possible she didn't cheat of course, but it seems incredibly unlikely. I've personally never gone to a birthday party for a coworkers 3 year old nephew a few months after meeting. I, also, have never gone on a family vacation with a coworker... hell I've never gone on a family vacation with a friend!
I think she knew how bad it would look and the questions that would arise if she suddenly had a new boyfriend so soon after her very public divorce (which was incredibly soon after her very public wedding to her long time bf) so she tried to keep their relationship hush hush. But then she likely accidentally got pregnant with Flynn, which forced her to announce their relationship and then her deep-seated Christian upbringing wouldn't allow her to have a baby out of wedlock, so they had that quickie "secret" wedding while she was still pregnant.
There's just too much weird shit and secrecy going on for their relationship to have begun totally innocently.