I really dig the illustration and the idea. Having the girl riding the bowling pin is super fun, and integrates the pinup style to the bowling team in a fun way. I like how you handled the colors, using just a couple and making a really striking and fun logo.
I like the shapes you used to draw her figure. I the like how the curves at her hips and torso and legs have this soft, rounded feel to them, while still keeping a clean, graphic style throughout. I like her pose a lot, too; it's strong and interesting and balanced with her arms propping her up, with her head turned as it is to make the pose more interesting and her expression more focal. I like her face a lot, and her hair, how they have that retro, graphic style, and how she looks really cute and fun. And I think the shapes of her foot/shoe are interesting, too!
I'm torn about the outline around her. At first it seems a bit distracting that it's rough and a bit uneven. I almost want it to be like...you copied the illustration of her and pasted behind and just made a clean white stroke around/behind her that would be smoother and more even. But then I look at the pin, and some of the shapes of her figure, and I like that there's this rougher, imperfect quality to the contours of the illustration -- I love the organic, lively look it gives to the art, so I don't think I'd want a cleaner stroke to work against that, either. But maybe if that outline was just a bit thicker, so it didn't hew so close to her silhouette to make it feel like it almost vanishes in a few places.
That said, I LOVE the illustration. It's so much fun and stylish and adorable and awesome. I think it fits for a slightly edgy bowling shirt PERFECTLY. As far as the illustration goes this is amazing. From a design aspect, I think it would've been fun to see the text integrated with the art a bit more. I think that font is a pretty good choice -- it's that bowling font, Singlesville? It fits pretty well. But it kind of feels like it's just there. I think it might've been interesting to do some cool custom lettering inside the pin, behind her, with the team name, or just find some way to integrate the text with/around the art a bit more cohesively.
But...I'm not that great at doing that; I work on designing (or reworking designs of) t-shirts at my job, and I don't always do a good job of integrating text and imagery into a cohesive design myself, so maybe take that with a grain of salt. But even so, I think there might've been a more exciting way to integrate the text.
But that said, I think this is an AMAZING design. I love the illustration -- I cannot emphasize enough how much fun, how cute and exciting and stylish it is. And even with my critique of the text, I think it's still a very, very strong design. I'm sure your pal will love it!