The first thing that comes to my mind is the voros "Why would a person with normal hearing wear a hearing aid?" If for the sake of pampering - then this is one thing, and if he doubts his hearing, then you need to check the hearing with a specialist. I think this question is from the category "What happens if you hit yourself on the head with a hammer?" - the answer "It depends with what force to hit." So it is with the apparatus. There are super-powerful hearing aids, which can cause barotrauma when wearing loud and harsh sound.
I will say this: as an audiologist, I often put on a hearing aid, which I tune, first on myself (with hearing, complete order). I listen to it in the version for minimal hearing loss. What for? To hear the difference in how hearing aids sound. Different models of different brands have nuances in shades. When demonstrating new models, we also listen to how they sound and let our colleagues listen (also with normal hearing). The sensations are interesting, but nothing critical.
Of course, if you put on, for the sake of interest, a super-powerful hearing aid that is tuned to a person with profound hearing loss, it will be very loud and unpleasant.
The hearing aid picks up ambient sounds using a microphone, processes them and reproduces them using the speaker. Modern devices are not just amplifiers, but rather complex electronic-acoustic systems with many settings. For example, they can amplify sounds of a certain frequency, suppress background noise or repetitive loud sounds, amplify speech, etc., making the sound experience easier and more comfortable. Accordingly, they need to be adjusted individually, depending on the results of the audiogram and the needs of a particular person.
If worn by a person with normal hearing, the effect will depend on the settings of the device. Since this is just an external device that modifies the stimulus itself without interfering with the processes of the body (like, for example, a cochlear implant), nothing special will happen, there will be a suboptimal and uncomfortable volume level of sounds - it's like putting on someone else's glasses, not specially selected for you - instead of making your vision clearer and more comfortable, everything will blur and it will be even more difficult to see.
By the way, even people with hearing impairments initially find the surrounding sounds and the sound of their own voices too loud, and wearing a hearing aid is unpleasant at first - it takes several weeks to get used to it (in this sense, it's not like glasses when immediately felt a sharp improvement). And no matter how modern and sophisticated the device is, it will still not be able to achieve the natural sounding of normal hearing without adaptations.
If you just put it on, nothing will happen. And if you put on a hearing aid that has just been removed from the ear of a hearing impaired person, then all the surrounding sounds will be much louder, as if you turn on the music louder. The volume level will depend on how weak the hearing is.