Pendulums work by channeling the subtle energies of the universe that your body picks up, and then communicating those energies in a way we can easily understand (ex. moving up/down is yes, side to side is no, etc.).
It's very possible that your pendulum has been affected by the other person's energies, and now your energies and their energies are conflicting with the pendulum. It's usually advised not to allow other to use/touch your magical tools, especially tools used in divination as this result can sometimes occur.
In addition to using sea salt or sage, you could cleanse the pendulum with sunlight or moonlight to remove the energies that have been attached to it. Be careful with using sea salt, as it could have a negative chemical reaction with some materials, depending on what your pendulum is made out of. Sage is a good option if you have some you can use, but I suggest sunlight/moonlight because they are free options and easy to use.
Just lay your pendulum out in the direct sunlight/moonlight and leave it there for 24 hours. Make sure that it will not be damaged by other people or by the weather. As a note, some stones/materials made fade in color with long exposure to sunlight, but moonlight is usually completely fine.