Agreed that there are folks who are toxic towards 5E in PF2E forums and that that's not ideal. Your partner is absolutely correct about that.
The thing to keep in mind is that that's not a solvable problem, at its root. Complaining about toxicity in online forums is like complaining that rain is wet - you can't stop it from being so, no matter how much you wish things were different. Which is not meant to be dismissive of your partner's accurate observation but moreso shift his mindset from "this is unfixable" to "what can I do to make my experience better?"
So if we assume that online toxicity is something your partner or you are unable to do anything about, what are things you actually can do?
Mainly, your partner can address how he reacts to that toxicity. Take on the mindset that, "These are internet strangers! Their opinions aren't truth!" And I'm not trying to say that he should completely ignore these people; some of them likely have valid opinions. In fact, he both can and should do his own research and, most importantly, develop his own opinions.
Someone says that "5E is just for auto-win stuff"? Okay, does that match his experience? Has he ever been challenged in his games? Seen a PC die? If he has then maybe that random internet stranger is wrong.
Someone says that WotC is a shitty company? Okay, do some research. Damn, they hired the Pinkertons to go after someone? Maybe they're not actually a company your partner wants want to give money to.
And so on. Maybe the simplest way to do this is, when your partner tells you, "I read someone being mean about 5e" you just ask, "Do you agree with them?"
I don't think he should "just deal with it" though: I think he should critically engage with negative comments, form his own opinions, and then trust those opinions over those of an Internet stranger.
Which maybe is functionally equivalent to "just deal with it"? Feels different to me.
Also I went to the Pathfinder2E subreddit, ran some basic searches meant to evoke comparisons to 5E, and grabbed the top result for each: "how do attacks work" ( https://www.reddit.com/r/Pathfinder2e/s/BBYlzCwVDl ), "advantage in PF2E" ( https://www.reddit.com/r/Pathfinder2e/s/Yna9TGzAOu ), "warlock equivalent" ( https://www.reddit.com/r/Pathfinder2e/s/umQ1Et6xhf ).
There are NO comments in ANY of those posts bashing 5e. On the contrary, there are dozens of comments that are all helpful, encouraging, and supportive.
So if you're looking for a place that is generally positive and welcoming to new players, r/Pathfinder2E has been pretty good in my experience! It's not perfect, of course. If you're looking for a knowledge center that a) has people posting and talking and b) never ever ever portrays 5E negatively I don't think that exists.
Anyways, hope your partner continues to enjoy ttrpgs regardless of the system!