I had a brainstorming session with a friend a year and a half ago, and a lot of great ideas came out of it. I moved on the ideas, starting a company - our product has been on the market for about 10 months now. I invited him to join me at the outset, but he declined.
In the last two or three months, he's launched a competing product with a lot of the same ideas. At the bottom, it says, "patent pending." I asked him if he was patenting any of the ideas that we talked about, or that my product has already implemented and released, and he was vague. "Friends don't litigate friends," he wrote.
So. Do I need to do anything? Can I find out what patents he's applying for? Is it too late to document that I had these ideas too? Is it enough to beat him to market with a feature, or can he patent something that he hasn't released yet?
Ok, here's my best answer (I have some grounds to answer since I was a target for a patent lawsuit):
How much money can your friend spend on useless litigation? If he does not have more than $100K in disposable income, forget everything about the patents and just proceed to build the best product you can, sign up the most customers you can and grow your business. You can safely ignore him and beat him fair and square where it matters, in the market.
In the unusual case where your friend is a rich idiot... I probably wouldn't change my tactics. The fact that your product has been out in public for 10 months is enough to establish prior art. Maybe just make sure you can prove that your product existed at a certain date.
You do understand that no matter what, even if he did file for a patent, it will take 2 to 4 years before he could ever sue you. Where will you be in 4 years? Most likely, your product will either be dead, or have pivoted to something so different that all of this discussion is moot.
I'm not going to give you legal advice here, because obviously this is a site for startups and not jurisprudence. But I would ask a few questions here.
First, just how novel is your idea? How sure are you that you and he are the first people to think of this? If you weren't the first (and no disrespect, you probably weren't), and someone has something resembling prior art out there, then it doesn't matter.
Second, do you believe he is really applying for a patent? He might just be full of it, and think that phrase on his page impresses people. It's not that easy to do, and it ain't free, so what are the chances he really is? I would venture to guess that any twit who would drop a phrase like, "friends don't litigate friends", has delusions about becoming Mark Zuckerberg or something and having Google buy his awesome product.
Thirdly, say you really believe you do have a novel idea, and really believe he is patenting it. Go straight to a patent lawyer, and get advice on this. Don't ask anyone else's opinion, go straight to an expert. He/she will tell you how to properly handle this, and if you could successfully litigate. Patent law is so crazy, amateur advice might screw up any chance you have of challenging him.
Most likely, the whole patent thing is moot; neither of you could patent it, someone already has the necessary IP and you couldn't afford to challenge infringers anyway. So stop worrying about it, and if you're a product guy and not just an idea guy, build the product and beat him. So what if he beats you to market; figure out his product's weakness and beat him. I don't mean to be trite, but end users buy products, not ideas for products.
I'm not a lawyer so I can't answer for any legal sides of this question. But I do have a couple of suggestions.