Yes, I believe you can. In religious belief in spirits at least from a Christian perspective about spirits being demons kind of goes like this;

Demons are fallen angels. Satan is a fallen angel it is believed that his job is to deceive those who don't know the 'truth'. It is often taught that these spirits and ghost are just demons in disguise. They will use the following verses to support this teaching;

2 Corinthians 11:14

No wonder, for even Satan disguises himself as an angel of light.

Galatians 1:8

But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let him be eternally condemned!

Ephesians 6:12

For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms.

So naturally if she is a Christian she will believe the scriptures she adheres to and the teachings of her leaders as to what they mean. As for you, beleive what feel right to your spirit. It is not necessary for anyone to take on another teachings or beliefs and especially if they are based in fear.