“Help yourself” normally means “you can take something without asking me” or “you can take something without asking me again” (if they already asked). To a houseguest. “If you get hungry, help yourself to anything in the kitchen.” (You don’t have to ask me) “I’m growing more tomatoes than I will ever be able to eat. If you want any, help yourself” (Just come over and pick some from my garden. You don’t have to ask me)

No you can't use it in that situation. You say help yourself when you want to allow the other person to take something from you. Like at a meal. Other examples... 1) Your neighbor comes to your house and asks if he can borrow the shovel you have in your backyard. You say "help yourself", which means he is welcome to borrow it. 2) You're chatting with a friend and she notices your library of books in the other room. She says "wow, you have a lot of books". You reply "help yourself if you want to borrow one".