This is all from my personal experience and knowledge database.
For one, your friend compared the damaging physical aspects of alcohol consumption to the damaging mental aspects of marijuana consumption. He starts off with his argument being flawed. The first thing that I would have pointed out is that if we were going to be comparing, then we need to compare physical downsides to the opposing drugs physical downsides and verse visa with mental downsides.
I'll give you a little background on my drug use, so you can see where my standpoint on daily marijuana use is and get a little info on the damage that I have observed, both mentally and physically.
-I smoke marijuana daily, and have for the past 5 years. I smoke weed from Cali, and I'm not just saying "Oh my weeds dank man, it's some cali bud..." It really is from Cali haha.
-I also take methadone 20 mg everyday, for the past month and before that I was on subutex for half a year. I am tapering and hoping to get off this shit. But let's not get started on opiates Haha :D
-I have consumed meth using many different routes of administration probably 60-70 times. NEVER used more than 4 days in a month and averaged maybe 2 days for the past 6 months.
-Around 3 months ago I took shrooms, at night, by myself. LOL not such a good idea but I ended up really bonding with my dad and I told him I was on them and he talked to me and kept me interested in different topics and made it to where I actually enjoyed it a little.
These are the things that may or may not have affected my psyche, for good or bad

.
I'm gonna try and be honest with you about my daily marijuana use. I'm gonna tell you:
-How I feel when I have gone without marijuana for 4-6 hours.
I can handle not being able to smoke for more than 8 hours without doing anything risky to get weed. I don't act moody and negative because I know I won't be able to smoke until I get home from work. It actually gives me something to look forward to, because almost every weekday I have to wait to smoke from like 2 P.M. to 10 P.M.
-How much I consume on an average daily.
I consume about .4-.5 a day. I have a 14 inch bong with a double perculator, and its the smoothest smoke I have ever had. The smoke passes through water three times... I might have to toke up before I finish this sucker...

haha.
-How I feel if I run out or have to go 12 hours+ without any. (I never run out and have to go more than 24 hours, and if I have it hasn't more than three 24-hour periods in the past 6 months)
When I have made it past 12 hours without smoking, I am really wanting to smoke a bowl, haha. I might be a little negative about things, get aggravated easily, do things that when I get high I will ask myself, did I really say this to him or her, wow I was being a dick haha.
-How I handle smoking my last bit of tree at night, and am either broke or just somehow know that I'm not gonna be able to get any the next day.
I feel like these nights are pretty shitty and I just have to accept that I can't smoke. I try to stimulate my mind doing other things that keep my mind off of weed. Preferably fun things, but hard work actually can serve the same purpose of keeping your mind on something. Your WORST enemy when quitting weed is boredom, remember that.
-If I have any hallucinations. I'm talking about perceptions that aren't there, whether they be visual hallucinations, verbal hallucinations, or even your sense of taste, smelling or touch, Haha.
Negative, and I sure hope that I never do!
Now, on to any negative physical damage I feel may have been done by marijuana.
I'll try and be honest here too, I'll tell you:
-Whether I have chest pains.
Sometimes if I take multiple rapid hits in succession, my chest will hurt for a little while.
-Whether my stamina has been affected.
Yes, my endurance has most definitely declined, from lack of exercise, and smoking.
To sum it up, I think that if you can control your use, marijuana is definitely a better option to choose if you are going to select a drug to consume and get fucked up off of, and want to keep the risk of mental dependence, mental degradation, or physical harm to your body. I think that alcohol, in large doses, can be damaging to your organs. As stated above, its not about the amount of administrations, its about how much you consume at one time. And judging by how you feel the next day, alcohol can't be good for you! Not that marijuana is good for you, but I sure don't feel like shit the next day off it, or even comedown.
I hope that my response is appreciated and of some help. Good day/night to you fellow Bluelighter!