Once you start asking those kinds of questions, you start down a slippery slope over what right a person has to another person's mind and thoughts. I don't see Solas as the type to decide it's all right to play around with anyone in that way, short of absolute necessity. Pain is a part of life, and while Lavellan can be sad and hurt it ended at least she was able to experience something incredible. Why take the good away just because some of it hurt?
I don't see Solas as the type to decide it's all right to play with anyone's feelings as well.... which is WHY I wonder he actually does things that look awfully like it (maybe not intentionally, but... eh, we know how intentions can lead to hell
)
I mean, let's face it - the hurt may eventually outweigh all the good, with the way things played out: Solas leaves Lavellan at a bad, emotional spot and then refuses to give her any means to move on.
He ends their romance at the most unexpected moment and we know through banter with Cole that she can't help but to think that it's because something's wrong with her. And why shouldn't she? Solas at first tells her that she matters to him and deserves to know the truth.... only to decide that she doesn't even deserve to know why he left her. That's a pretty bad punch into girl's self-esteem right there.
He exacerbates it by telling her later that they have more important things to do and seems to treat her like a child, rather than respected adult he apparently had her for till the breakup - no matter how strong psychologically Lavellan is, I'm surprised we don't hear her heart crack loudly right then and there.
It's especially sad to see if we then go and talk with others and what they do is basically say their farewells to her - everyone and their dog basically expect her to confront Corypheus and save the world or die trying. Her feelings on it don't matter, and the guy she thought she can lean on emotionally just pulled a rug from under her and built an ice wall between them.
Even worse, he promises her that she'll find out what that was all about after fight with Corypheus... only to disappear right after the fight's done. He assures her that what they had was real, true, but like I said - it will just make it harder for her to get over him and smells awfully like playing with her feelings, especially if (as we know through Cole) Solas likely doesn't expect to come back or involve anyone he met along the way into whatever he's supposed to do from now on.
IMO Lavellan has the right to feel used and emotionally abused. It'll be even worse if she finds out who he is.