Talk it out.
Address the issue by talking to the person privately. "I felt betrayed that you used my idea for your pitch," is loads better than starting a rumor that your colleague is dating the boss. #immature
Keep things professional.
You and your colleagues do not need to be BFFs just because you see each other every day. You can limit your conversations to work-related stuff, which means you get tasks done faster and with less complications (read: emotions) involved.
Inform your HR or your superior.
Before the situation gets out of hand, it is important that you tell your boss about it. Ask for guidance and let him or her know how you feel, but don’t make it sound like you’re nagsusumbong. The last thing you want to happen is for a small problem to get blown out of proportion.