What rating should a coach have
I think any coach should have a base level of competence whoever he/she is teaching. Where that base level is, I'm not sure. I do know players under 1000 rating blunder pieces all over the place - I don't think these players should be teaching chess (other than the basic rules) to anyone.
I've noticed a number of threads lately where very low rated players are offering coaching. It seems to me that if you're going to offer lessons in pretty much any activity then you should be able to demonstrate some sort of competence in that same activity. Now, I'm not saying chess coaches all have to be GMs or anything, but surely they should have a rating in four figures in order to help even the lowest rated beginners?