Watching her dreams is not the same as watching her constantly and seeing what she does IRL
Also - wouldn't that jeopardize all of her plans of trying to at least slow him down, while she searches of a way to change his heart?
I think it's implied that, as a Dreamer and powerful ancient elf, Solas does have access to minds of others through dreams, but he doesn't abuse that power. He's too principled for that (for now, at least...). He felt familiarity with Blackwall, but didn't know of his past as Thom Rainier, for example.
Therefore, while I think he sometimes just can't help himself to not be drawn to Lavellan - or she to him - while they dream (I think there's something going on that Cole once described to Iron Bull - Bull's pain and care for Tamassaran he viewed as mother figure 'connected' them both, which is why Cole could 'read' Tamassaran's pain and worry over Bull), but doesn't directly invades her dream or observes her like Edward Cull... I mean, like a creeper does 