If it's an actual demon of lust then it's not really contagious, unless he let the thing in - in which case some kind of exorcism is needed. If it's just some sex-adjacent elemental spirit then it really depends how you like to govern your inner world. I would get rid of it by invoking an opposite current of power.
Could he let it in without trying to?
Possible, to a degree. One can agree to things without knowing their consequences. Sometimes spirits exploit vulneralibilities created years before. Depends on what kind of spirit we're speaking about. What symptoms does your friend notice?
He's noticed a new desire to have sex as a way to conquer or take from others, after sleeping with the girl he looked her in the eyes and felt like a primal fear as if she was demonic. It's worth noting that they were both virgins and the sex was unprotected, it also happened on the 6/7th of August which was fairly significant since it was just before the eclipse. The girl was into witchcraft also. He deeply feels that the girl looked possessed and felt intense dread when looking at her eyes. He says that everything was going well for a few hours after the sex.
