As someone who went from really liking Solas to being super pissed at him after Trespasser, I can sort of understand how your friend feels because there ARE people who seem to bend over backwards to justify his actions for the love of the character. Or at least that's what it looks like to me. Anyway, it is also possible to be a Solas fan and keep his both his flaws and good qualities in perspective. Although I am angry with the character, I am still hoping that he's redeemable.
It is possible to empathise with and understand a characters plans from that characters perspective, without agreeing with them.
I completely understand why Solas feels he needs to carry out his plans, even though it hurts him to do so. I understand how utterly wrong modern Thedas must feel for him. It's a mistake he feels he must undo. To do nothing would be, from his perspective, a betrayal of the world as he feels it should be.
That does not mean I agree with his plan. But I understand it. And I also think there's more at stake here than we've been shown as yet.
Imagine if you would, that instead of the time magic during the 'In Hushed Whispers' quest casting your inquisitor forward one year, it cast her forward a decade. The world is broken, demons are everywhere, everything is all wrong. Now imagine that instead of being able to fix it instantly, your Inquisitor had to spend a year there, gathering the resources she needs to turn back time. During that time, she meets children born after the breach. She meets resistance fighters who, for all the hardship they've endured, have formed bonds of friendship and love that they never would have had if not for the chaos. None of these people would want those things erased. In the case of the children, none of them will even exist if your Inquisitor turns back time. She's erasing them. All because she sees the world they were born into as broken.
Is it broken? Well, yes. The Inquisitor knows that, because she lived in the world before it broke. But does that mean anything to the children she's going to remove from existence? Does this broken world not have the right to fight and mend itself on it's own terms? We've already made that choice for it once. We chose to erase it. Who knows what it could have become, had we chosen instead to stay and help fight.
Returning to Solas; is modern Thedas 'broken' in the same way that the breach future was? Solas certainly seems to think so. And how can we say otherwise? We have no point of reference, nothing to compare it to. We weren't there in Arlathan. We have no idea what was lost. Solas sees modern Thedas in the same way our Inquisitor saw the alternate future. Broken. She made her decision, and that world ceased to be. Now Solas has made his.
'Let he who is without sin cast the first stone.'