Bad idea, LSD is just another psychedelic although I find it less ego-destroying than mushrooms. A cannabis overdose (yes it's possible even if it is not physically threatening) is almost a reliable way to make you feel like you are dying isn't it

I think that is the danger with something like JWH-018 as well. Cannabis is not to be treated like there is a ceiling effect although normally when you are smoking it with tolerance that can prevent you from overdosing.
Eating spacecake though can make you feel like you're dying, and waiting until your tolerance is gone and then smoking a lot of dope can too I guess.
I have had sort of traumatic experiences with LSD + nitrous that really scared me so much that every time after I wanted to try it I got terrible fear.
What helped with that was MDMA actually, MDMA definitely removes fears and can be therapeutic like that although it can't be a life-time guaranteed fix.
If you are thinking about drug-assisted therapy then I think MDMA is a way better option than LSD.
While on the MDMA it is good to review those other experiences and try to relive / remember them a little to see what actually happened there so that you can process it.
After that seems to have helped (or it might just be a wonderful roll and that's about it) when he decides to try tripping again LSD may be an okay option, at least better than mushrooms and better than smoking more weed than you can handle.
Even better if you can get it is lighter psychedelics like 2C-B, 2C-C, perhaps 4-HO-DiPT, and start low (with LSD as well)!!
Benzo's are always good to have on hand with these sorts of things. Lorazepam is a good panic attack stopper but Diazepam or Alprazolam and many others work perfectly well too. Some are somewhat less suited though and can put you to sleep just as fast as helping your fear/anxiety.
BUT: don't take a benzo when shit gets just a
little rough, that way you always go around the issue and there will hardly be anything therapeutic to speak of. And when it is time for a benzo, take half a dose at first rather than a whole one. It may take the edge off while still allow you to confront the experience.
If all these sorts of things don't work, tell your friend just not to trip again - it's that simple. If troubles with anxiety or panic possibly remain while sober then therapy is your best bet. And they might help you and if necessary medicate. Medication is generally not good unless/before serious therapy has been tried.
And HPPD is not the right term here, HPPD is when you see things like static and get other weird trippy effects without having taken something.
A much better term if you must use one would in my opinion be PTSD (or a mild case of it) caused by the ego-desintegrating effects of psychedelics; with the intense experiences he had with them.