Yes, i am pretty sure you are dealing with a demonic enity. Considering what it has done already it may not be as easy to get rid of. Just banishing it may not be enough. If it can cause seizures it is most likely very old or could be a spirit that is protecting itself as a shadow person, there are a couple reasons why i beleive it may be a spirit.
1. It is usually not known for shadow people to pray on middle aged adults. But shadow people mess with children and elders. Usually tricking them into doing dangerous things
2. I havnt heard much about shadow people causing seizers. Although it may be possible. I strongly advise you and your friend to contact a physic medium. Preferably a strong willed one to give you more information. You are definitely dealing something dangerous and i advise you not to take it lightly. For now, best wishes.