Question about new menu feature in v3

This is not a coding question, just need to know if I was given right information about new menu feature.

The original theme uses drop down menu for the category menu. It’s based on new menu feature and it’s the main menu on the site below the logo. But I had a guy change it, so instead of drop down it lists sub-cats horizontally underneath parent cat menu. Also, when I go to sub-cat or single post page both parent and sub-cat menu items remain
active. When the guy changed it he didn’t do it through nav_menu feature, sub-cats show up automatically for that parent category. Through some code in header and function file. The problem is I have no control over order of sub-cats. When I asked a friend of mine if it’s possible to implement menu with my setup, basically do what it does now but can be controlled through menu feature, he said it’s
impossible because new menu doesn’t specify something about sub-cat to be able to know when and what menu is active. I don’t know much about WP to know if its true or not, but I figured I ask community to see what some WP pros might say.

Right now, half the category menu is controlled through menu feature and the sub-categories display automatically. If I add them inside menu feature dropdown activates on top.

Would appreciate your feedback.