I'll give my personal thoughts on it. I identify as a masochist and a submissive. I have in the past enjoyed what some might consider edgy play and tend to do so more under the RACK(Risk Awareness Consesual Kink) train of thought than SSC (Safe, Sane, Consensual). My reason being is because a lot of what I enjoy very well might not be consider safe or sane by the throngs. Of course, neither is driving. I tend to know what I like, research it, try it with someone who is knowledgable and in doing so am more able to be aware of the possible risks involved.
That said when/if I go to subspace it is usually in one of two ways. The first is brought on by a sensation I really, really enjoy. For me this would be something thuddy usually combined with sensual/sexual touches and hopefully some sort of mental encounter as well. Words from my partner usually do this best. When I go in this way it is a place that is very peaceful. I relax into every sensation... think of that relaxed place you go during a really good massage. there is no tension in my body. I am a boneless heap and enjoying every touch, every stroke of the flogger or whatever impliment. My partners words are all I hear even in a crowded room. Everything sort of blends together and is all just really great sensation. Aftercare for me in this instance is not really necessary with the exception of making sure my head is on straight and not leaving me alone until I am more "awake". I am happy, smiling, laughing etc when I come back and enjoy the hell out of hugs. The "I love everyone" mentality is what I feel and it is nice.
The other way I get there is not as nice or as easy. It is usually brought on by a sensation I don't process as well, stingy things would be it for me. In this case I enjoy the sensations until a certain point and then I fight it. I hate to be emotional, especially cry in front of others so I will stomp, cuss, fight my binds if they are there, do pretty much anything not to cry. Then often times comes the tears, huge sobbing gulps which is very unusual for me. After that I am in this wonderful place of beautiful swirling colors and quiet. My mind is no longer racing and every sensation is positive.
This is the time that I need aftercare. I usually get the shakes very bad when coming back to myself and so having arms around me or hearing a soothing voice makes all the difference in the world.
I know it sounds harsh and ugly but if I am allowed the preference of which way to get to subspace this would almost always be the one I would chose. I tend to hold in my emotions and after this I feel absolutely reborn. It is a hard thing to explain, really.
This is also the reason why I don't play in any way that is edgy casually. There is a great amount of trust involved. I need to know that my partner understands my body language, my tears and that he will be there to take care of me after the fact.
I have not felt any long term damage as a result of going into subspace, even deeply. As a matter of fact, it has given me more freedom to be me, to enjoy being a masochist and a way to express myself emotionally.
My advice would be to ask your friend what they think about it. How it feels to them and their reactions before, during, after. Are they cared for after. Are their limits being respected? That type of thing. maybe it will put your mind at ease.