Whether Charlie can be convinced to take such a pill depends on whether he has other values besides the one specified (not being deluded) which can realistically compete with that one. For instance, perhaps he cares about his family, who are harmed by his depression.
Greg Egan's short story "Axiomatic" is close to the first scenario. Complete synopsis in rot13:
N zna, n pbzzvggrq cnpvsvfg, unf n tveysevraq jub vf fubg nf n olfgnaqre va n onax eboorel. Gur eboore vf pnhtug naq pbaivpgrq ohg trgf n fubeg fragrapr. Gur zna jnagf gb xvyy uvz, lrg vf nyfb bccbfrq gb xvyyvat uvz. Fb va beqre gb or noyr gb xvyy uvz ur ohlf na vyyvpvg qeht gb ercebtenz uvf trareny ivrjcbvag gb bar bs "Crbcyr ner whfg zrng. Gurl qba'g znggre." Gura ur tbrf gb pbasebag gur eboore, abj bhg bs wnvy, ohg orsber fubbgvat uvz, ur nfxf jul ur xvyyrq uvf tveysevraq, naq trgf gur bss-unaq nafjre, "url, fur jnf whfg va gur jnl, zna". Gur eboore unq gur fnzr nggvghqr gung ur unf whfg chepunfrq. Ur wblbhfyl rzcgvrf uvf tha ng uvz, abg va eriratr sbe uvf tveysevraq, ohg orpnhfr [crbcyr ner zrng, gurl qba'g znggre].
Gur qeht bayl unf n grzcbenel rssrpg, ohg gur fgbel raqf jvgu gur cebgntbavfg vagraqvat gb trg n irefvba gung jvyy znxr vg creznarag.
So, what do you do with Gandhi after his viewpoint has changed and he's done the deed? What does Gandhi do with Gandhi? I think this is a case where hardening the problem by elevating the stakes obscures the issue rather than focussing it. Just about any means can be made to look justified by making the ends important enough.
Slight nitpick on your summary of the story:
Gur cebgntbavfg qbrf abg rzcgl uvf tha vagb gur eboore zreryl orpnhfr crbcyr ner zrng naq qba'g znggre. Vafgrnq, gur cebgntbavfg unq vagraqrq gb yrg gur eboore yvir orpnhfr gur cebgntbavfg ernyvmrq gur jubyr fvghngvba jnf nofheq naq abguvat znggrerq nalzber, abg rira uvf tveysevraq'f qrngu (vg'f nzovthbhf nf gb jurgure gur qeht jnf gur cevznel pnhfr bs uvz pbzvat gb guvf pbapyhfvba). Gur cebgntbavfg ghearq gb jnyx njnl, naq gung'f jura gur eboore ehfurq uvz. Va ernpgvba/frys qrsrafr, gur cebgntbavfg fubg gur eboore. Nsgre frrvat gung ur jnf qrnq, gur cebgntbavfg sryg ab erzbefr naq yrsg.
It feels odd replying to a 4 year old comment, but I am simply too curious as to why all that text in written in what at first glance seems to be random assemblies of letters in the format of whatever Greg Egans story was
If Charlie's depression is that central to his 'self', then taking such a pill amounts to suicide. The existence of a post-pill alternate Charlie shouldn't matter; it won't be him.
The parallelism between Gandhi and Charlie here is poor because Gandhi has strong, externally directed terminal values, and Charlie doesn't. You could strengthen the problem by making Charlie a passionately committed negative utilitarian like Sister Y.
From a human perspective, other people have a right to (what seems to you) stupidity as much as they have a right to anything.
To assert something like "you know what's best" is to assert omniscience, which if true wipes away all ethical questions when you can just ask "what is the best course of action?" and take that.
If Alex has a talent for painting, but a utility function based on how much good music is composed, and Beth has a talent for composing music, but a utility function based on how many good paintings are made, they will want to trade terminal values.
Gandhi is the perfect pacifist, utterly committed to not bringing about harm to his fellow beings. If a murder pill existed such that it would make murder seem ok without changing any of your other values, Gandhi would refuse to take it on the grounds that he doesn't want his future self to go around doing things that his current self isn't comfortable with. Is there anything you could say to Gandhi that could convince him to take the pill? If a serial killer was hiding under his bed waiting to ambush him, would it be ethical to force him to take it so that he would have a chance to save his own life? If for some convoluted reason he was the only person who could kill the researcher about to complete uFAI, would it be ethical to force him to take the pill so that he'll go and save us all from uFAI?
Charlie is very depressed, utterly certain that life is meaningless and terrible and not going to improve anytime between now and the heat death of the universe. He would kill himself but even that seems pointless. If a magic pill existed that would get rid of depression permanently and without side effects, he would refuse it on the grounds that he doesn't want his future self to go around with a delusion (that everything is fine) which his current self knows to be false. Is there anything you could say to Charlie that could convince him to take it? Would it be ethical to force him to take the pill?
Note: I'm aware of the conventional wisdom for dealing with mental illness, and generally subscribe to it myself. I'm more interested in why people intuitively feel that there's a difference between these two situations, whether there are arguments that could be used to change someone's terminal values, or as a rationale for forcing a change on them.