What if a neutron star was hit with an antimatter star, which is half as massive as a neutron star? Could a neutron star lose mass and become a white dwarf?
Consider, first, the highly explosive nature of antimatter, with one kilogram of antimatter having an explosive force equivalent to a 43 megaton nuclear bomb when combined with an equivalent mass of antimatter. If a half-solar mass of antimatter combined with an equivalent amount of matter, the resulting energy release of up to 1.8×10e47 petajoules, equivalent to that released by hundreds of thousands of supernovas. I would be surprised if anything remained of that neutron star; I would be surprised if the galaxy where this calamity took place looked the same afterwards, even.
An object with half the mass of a neutron star could be upwards of 1.5 solar masses. Without even doing the math, I can tell you that the energy released from that much antimatter meeting regular matter would utterly annihilate both colliding bodies and anything within their star system. If we noticed it on Earth, it would be in the form of the most intense gamma ray burst recorded in all history, past and future; cataclysmic doesn't begin to describe the magnitude of such destruction.