Really good question. You can do it via one of three ways:

1) Provisional Patent. if an implementation of their system to his new market would be patentable (i.e. using machine X to make donuts instead of jewelry), then you could file a provisional patent on the idea. Provisional patents are very informal and cheap. They can essentially be written on the back of a napkin, and filed for ~$100. They last for only a year, after which you have to convert them into a 'real patent', otherwise your idea will become open to the public to use.

2) An NDA. You could have them sign a non-disclosure agreement (NDA), which would have wording in it that basically says, "I'm going to tell you something, and unless you have existing proof that you already had thought of that idea, then you're not allowed to use my idea without my consent". Sometimes the company will not want to sign an NDA because they may have discussed a lot of ideas without writing them down, and by signing your NDA, they'd legally lose the right to use one of those non-written-down ideas. But it's worth a shot.

3) Rely on trust. The way to make this more likely to work is if you are good friends with a big investor in their company, or something like that. Some situation in which, if they steal your idea, they'd be ruining their reputation with someone they care about (they probably don't care about pissing you yourself off). If you don't have such a connection, maybe you can form one through linkedin.

If you'd like to discuss any of this further let me know,

all the best,

Lee