I agree with this. Just think about it from his perspective. He is the HC and this other guy who he may of thought offered to "help" out at practice is telling him he is doing everything wrong and how he should change the way he is conducting practice after 20 minutes of being there. I know I wouldn't be pleased about that. I probably would have asked you to either help out or leave.
The best place to have these types of conversations is outside of practice. Give the guy a call or ask if you two can meet on a separate occasions with just a few girls to go over things. But to spring this on him during practice is probably shock and a WTF moment, regardless if his teaching methods were incorrect (which they definitely are).
I would not suggest that you just show up and ask him to coach the first 20 minutes of practice while he observes you. That seems a little insulting and I am not sure he would take that very well. There is clearly a miscommunication. You need to have a talk away from the field and explain your intentions. If he would like your assistance then great, if not then it is what it is.
Thanks for all the suggestions and views I had not thought of. I will contact the HC before the next practice I can attend and discuss my role, limitations and possible practice plans going forward.
If I'm not comfortable with it I will wish him good luck and move on.