How old is he? Zoloft is an SSRI(selective serotonin reuptake inhibitor) and Xanax(alprazolam) is a benzodiazepine(anti-anxiety). 100 milligrams is a moderate amount of Zoloft, how many milligrams of xanax does he take? How often? Pot is considered a part of the hallucinatory drugs, but its effects(as well as most drugs) depend on the amount used daily, abuse, age, etc. Does he smoke alot?
Zoloft and Xanax is not a bad mix. Neither is Zoloft, Xanax and weed. But this is speaking it unabusive terms. If he does this to chill out every now and then, and if he suffers from GAD OR SAD OR Depression, the guy rightfully deserves too. But taking Xanax to sleep is not a smart plan, it works great(when used passed 1.0 mg or higher), but continued use leads to physiological addiction- his body will begin to get used to falling asleep with Xanax(this is with continued use of daily adminstration of the drug for sometime). Xanax effects GABA neurotransmitters and if abruptly stopped can cause withdrawal symptoms. This also has occurs with heavy milligrams of the drug.
Do not get my wrong and think that I am saying he is addicted or abusing the drugs. People who don't use drugs are going to tell you not to use any drugs and all the negative things about them(without any empirical knowledge- through experience). But beware of others who are in love with drugs and tell you grandoise things about them. Just try to learn something from each.
Just keep an eye out on him though- if you see it is starting to have dentrimental effects, thats when you have to start to research and come up with effective plans. Do not worry or panic, it will not help at all, rationality is key.
hope this helps.