Honestly though, thinking about this whole thread a bit more, is LFR really the best place to try and teach a complete stranger how to play their class? I would argue that it's probably one of the worst places. People in LFR want to get through the place quickly and without a fuss. Giving them a running tutorial will slow both of you down and make both of you perform worse and really is very unlikely to achieve anything useful or helpful.
There is a world of difference between explaining fight mechanics and tactics to fellow LFRers and teaching them the basics of how to play their class/role. The responsibility for that lies with people on their own realm, preferably guildies and friends. Not strangers in a LFR instance.
This sort of thing should be done after an LFR run, and unless the other person is on your realm (or a connected realm), it would be completely pointless. I honestly doubt the OP's sincerity about his real intentions with the other warlock.
I still remember many years ago when I first joined a raiding guild and started to raid properly. There was a really awesome hunter in the group at the time (well I certainly thought at the time he was awesome!) who decided to take me under his wing.
After about 2 weeks of raids, he sent me a private mail that started something like this:
I have been watching how you play and would like to say you're not a bad hunter. However there are a few areas I have noticed where you can improve..."
He then proceeded to produce a short essay on what I was doing wrong and things I should be working on to improve. The "truth" is I was a noob and I was not very good at all. I had a great deal to learn and grow before I would become a competent raider. But because of the way my mentor elected to approach me, he got himself into a position where I could learn from him. And I did.
Offering unsolicited "advice" to a complete strange who you are unlikely to ever see again in a setting like LFR is a recipe for disaster. OP really shouldn't have been surprised that his advances would be met with suspicion.