Some thoughts from me on this specific point (I'll spare you any further comments as I really don't have the time to unpick all this):

Firstly, a nit pick: This friend of yours has not done any proper theological study on angels and demonology. Not all angels serve God. Lucifer fell from heaven and took a load of angels with him (Isaiah chapter 14, Ezekiel chapter 28). Angels, fallen or otherwise, are not the same as demons. The origin of demons is a mystery in the Bible, but the Book of Enoch (not in the Bible, but clearly an important Jewish text, since the apostle Jude quotes it Jude chapter 1 verses 14-15) reveals them to be spirits of the Nephilim - human/angel hybrids slain, whose restless spirits wander and seek humans to possess). Therefore, not all angels are good in any case, and angels and demons are not the same as departed human spirits.

I'm not necessarily saying ghosts are real - please don't take that away from what I'm saying - but the Bible does have a few inconvenient passages that at least throw some mystery into this, rather than the cut and dried binary perspective mentioned here by your friend.

Near the start of the Old Testament, in Genesis chapter 4, we get the story of Cain and Abel. When Abel is murdered by his brother Cain, the Bible speaks of Abel's blood "crying out" to God. This is the first ghost story, and one that sets the tone for many subsequent ghost stories that often concern restless spirits seeking justice for wrongs.

Additionally, in 1 Samuel chapter 28, the ghost of Samuel is apparently conjured by a witch at the behest of King Saul. There can be no doubt at all, in the context of the story, that this is really Samuel, not a demon.

In the New Testament, Jesus also seems to have knowledge of ghosts and folklore. Even if he doesn't state outright that ghosts exist, he discusses them not having bodies, flesh, bones, etc (see Luke chapter 24 verse 48, and Matthew chapter 14 verses 26-31). Then there's the weird appearance of Moses and Elijah during the Transfiguration (Matthew chapter 17 verses 1-9, Mark chapter 9 verses 2-9, Luke chapter 9 verses 28-36).

On top of that, the apostle Paul speaks of "a great cloud of witnesses" in Hebrews chapter 12 verse 2. As someone who has lost their father (who was also a Christian), I won't lie: Sometimes I've sensed his presence, even though I have absolutely no concrete proof of this. And sometimes, a lightbulb will flicker. I've even had messages from him in dreams, some of which have unlocked previously blacked out memories (that's a very long story I won't get into here). My theory? I personally believe that God sometimes lets this "great cloud of witnesses" - including my father - pass on messages into this dimension, or even lets them take a look at what we are up to. When I pray, I sometimes ask God to pass on messages to my father too. That will sound foolish to many, I daresay, including some of my fellow believers. But there it is.

Here's a satirical fantasy short story I wrote inspired by this subject. I hope you'll give it a read, as I suspect you'll find it entertaining. :)

https://medium.com/fictions-official/in-between-9631093dc799