The max friendship level of cowgirls is 9, and you get it by asking all 4 questions, feeding the favourite food, and getting each body part to the max level (which is 7 for all parts except for boobs, which is 9). There is a system to when body parts increase in size, but if you just feed the cowgirl until you can't anymore, that should do it. A heart will appear above the cowgirl when the friendship level is about to increase, and if you talk to her, it will increase the friendship level.