From the Pixiv preview from Chapter 1, I think the whole story goes like this:
1. Boy hangs out with Girl, Girl found out he was actually a boy.
2. Girl previously had 0 friends, and she told Dad she made her first offline friend ever.
3. Dad was overjoyed and wanted to meet this 'friend', threatening to beat him up if he happened to be a dude.
4. So Girl begged Boy to crossdress.
5. He accepted, so they became closer as friends.
6. Boy is also well-versed in make up, so it might be that he also wanted to crossdress sometime.