Is my friend too big for my horse?

If you are uncomfortable with letting her ride, then you should follow your gut feeling and not let her. It's probably best to tell her the truth, even if it hurts her feelings. That way she won't ask to ride everytime she comes to your house. Keep in mind, she knows she's big, there's no way she couldn't. So if you say, "I really shouldn't put more than 150 pounds on him. It could hurt his back." she'll get the picture. If you don't want to tell her the real reason, you can always make up a little lie.. "He's not feeling well today." or something along those lines.