CTL – While I hear your point, I would phrase it slightly differently. I do think it’s important to acknowledge the fact that the people in question do keep some things and therefore, I understand why the term “non-observant” bothers you.
However, I do have an issue with phrasing it: . “He observed the law according to the rules of his movement.”
The movements are kefira. Rav Moshe Feinstein makes a big distinction between a “Conservative” Jew and a Jew who acknowledges the truth of Torah and Mitzvos but doesn’t keep them. While I realize, (and I think Rav Moshe pointed out) that the Conservative Jew may not know better, he is still an apikorus (albeit unwittingly). I am not coming to judge him in any way, and it is wonderful that he tries to keep all he does, but I don’t think that WE are allowed to make a statement that in any way lends credence to the movement.
I realize that was not your intention, but I think that it would be better (and more emesdik) to state that he is not non-observant since he does in fact observe some of the Mitzvos of the Torah (without mentioning his Conservative affiliation – that is not a maaleh for us – what is a maaleh is the fact that he keeps some Mitzvos despite being Conservative).