I think I made a mistake about a month ago and disclosed an idea that I had to a colleague. I'm in the process of taking the idea to market, but it hasn't gone public yet. I wasn't planning on patenting it, but it very well might be patentable. Can the person that I disclosed the idea to patent it?
At least in the US, part of filing a patent application involves an oath/declaration, which includes a provision such as:
Filing such a claim if he knew it to be false (i.e., knew that he was not the original and first inventor of the subject matter) would not only invalidate his application, but is probably1 illegal in itself, and could be subject to prosecution.
There are some provisions to allow (for example) a patent application to be filed if the inventor dies before filing the paperwork, but it doesn't sound like those would apply here.
As another answer says, only an actual inventor or someone an actual inventor has assigned their rights to can legally file in the U.S. since the AIA went into effect the declaration has been changed to
If the disclosure would not be considered confidential it did start a one year clock on when you can file. Also, as mentioned, if you both file and he files first you need to prove he derived it from you. Maybe worse, he can invent something different, inspired by what he leaned from you.