Even if you don't need vision correction, a prescription would still be required. This is for your own safety and the health of your eyes. You may have read some news on how special effect contact lenses are not safe and you should not wear them. This is why a prescription is important. It can mitigate risks associated with wearing Halloween lenses.
All because contact lenses may be for novelty purposes only doesn't mean that the same care and consideration shouldn't be taken as with regular lenses. In fact, it is illegal to sell special effect contact lenses in the US without a valid prescription. There is no one-size-fits-all approach with contact lenses.
Once you then buy safe contact lenses (FDA-approved in the US) from companies who request a prescription, then you just need to follow the proper wear and care instructions. You then shouldn't have a problem with your eyes and can enjoy your Halloween costume in full force, without worrying about any health risks. And no, your friend that asks you to borrow your cool contacts should not be wearing them.
Follow this safety advice for Halloween contacts and special effect lenses year round and you can enjoy enjoy getting cheap novelty lenses time and time again.