Is a person a partner in sin if he brings a device to its owner, knowing beforehand that its owner sometimes uses it for bad things, and then its owner uses it for a bad thing?

It is not permissible for a Muslim to assist in a sin. If a Muslim knows that the person he is helping to acquire a device will use it for evil, then it is not permissible for him to acquire it for them. If he is not certain of this, then the preponderant assumption takes the place of certain knowledge. A suitable remedy for such a situation is to strive to guide this person, advise them, and remind them that these devices are blessings from Allah Almighty, and therefore should only be used for what benefits one in their religion or worldly life.