Hey Dicey, my perspective is that the non-pejorative uses of the word queer is a generational thing.
I was born in 1980. My world was formed by Ryan White, Matthew Sheppard, Ellen DeGeneres. Gay culture was confined to the big cities. As a kid I knew about funny men because of the AIDS epidemic. I didn't women could also be funny until 1993, when Ellen DeGeneres caused such a stir. The state mandated sexual education at my school was strictly about how babies were made. Anything extraneous to babies was left out, and I had no idea women could have orgasms until one very surprising day in the early 90's. We played smear the queer at recess, the populace called for ghettoization of anyone with AIDS. There was no Gay Straight Alliance. The internet didn't really exist. Don't Ask, Don't Tell was seen as a progressive and merciful policy. "That's so gay" rang through the halls of my highschool to note anything stupid, dumb, or ugly. I was tainted. It was isolating. I wanted to die. Hearing the word queer from a heterosexual person could never be anything besides an epitaph, and it was rarely positive even from a homosexual person.
Adults born 10 or 15 years after me had a completely different experience. People with HIV lived. Will and Grace was on mainstream TV. Queer as Folk, and the L word were on the edgier channels. Gay culture cropped up in smaller cities, even towns. Civil unions were declared legal. Gay Straight Alliances were formed in schools. Safe spaces. Sexuality was a discussion people were having. The internet provides support and community. To these slightly younger people, the reclamation of the word queer was a powerful thing. To a half generation younger than me, queer is a word that's sometimes negative, and sometimes positive, depending on the speaker. Much like the reclamation of racial pejoratives among the subjugated class.
Now, the word has dropped even more of it's negative connotations, thanks to the transgender and pansexual movements. The boxes for gay, straight, and bi-sexual are too confining. Especially when viewed through the idea that allowing for only two genders is equally confining. If you happen to be a gender fluid young person, and you're making eyes at someone who was genetically born male but is taking androgen blockers because she definitely know's she's a fabulous girl...well, what is that? What word could you use to encompass what you're feeling? You aren't gay, and you aren't straight, but you can certainly be queer. So now, for people born 20 to 25 years after me, queer is just what they are. Sure, sometimes the word gets miss-used by assholes, but fuck that noise.
So, that leaves us with your question: if a person who identifies as heterosexual called someone who identifies as homosexual "queer" that it was considered a slur. The answer is, it depends. I shudder when I encounter the word queer; MonsterMonster embraces it. Different people, slightly different generations. As with many personal things, the best thing to do is simply ask the person directly their opinion.
To close this wall of text, I'll point out that your question is very obviously from a standpoint of curiosity and a desire to learn. Thanks for being cool.