It isn't assured to give him peace of mind, as I said it sends many people down a path of questioning if the doctor is wrong and seeking second opinions, and if they have an ekg and the automated interpretation says "q waves present" and the doctor says it's nothing just a little artifact, then they google and find that it could indicate ischemia has happened in the past, then they doubt their doctor and their anxiety is worse and they keep looking for someone to validate their concerns.
It does not always give peace of mind, for many it's better to let it go than to go down that path.
I have seen a lot more of this than you can imagine. This is so run of the mill that it's almost all of the posts in /r/ekg - healthy, normal ekgs by young people freaking out about things they don't understand and don't believe when they are told it's normal.
This isn't reckless. As I told him if his heart rate is fast or very irregular, that's an objective sign. If it's 110 and the layman says "oh that's high because 60-100 is normal," that's bad advice. It's a normal variation. I said he can. Ring it up with a GP at a regular appointment/physical bit it is not by itself a reason to go and it is certainly no reason/indication to see a cardiologist.
In the absence of overdose on hard drugs, there is no precedent for someone 21 years old with "a few years" of drugs and alcohol to have any cardiac problems so early on. Congenital heart problems are usually caught very early on because they make themselves readily apparent (and when they don't, they don't cause effects for a very long time - this week I had an 81 year old diagnosed with a heart shunt that was present at birth and just now causing problems).
Anyways I'm rambling but the reddit layman brigade always have good intentions but always lack knowledge and sometimes their advice is either counterproductive or unreasonably cautious.
Plenty of 20ish year olds show up in our ER for cardiac symptoms/chest pain. None get admitted. It's very common and it's psychosomatic.