We let DD 0ccasionally do gym friend sleepovers on nights before practice if it's a later start (like, 11am or later), mostly because they all "get it" and go to bed at a decent enough time.
If it was for a rare sleepover/party that DD *just had* to go to, I'd consider giving practice a miss, depending (like, probably not the week before a big/first meet). That said, DD rarely misses practice for even optional school events, much less sleepovers (says she wants to practice).
If my DD had a long-awaited private, I'd probably want her to skip the party. That sounds like what you chose? I get letting kids be kids... That's important and as a mom I'm mostly flexible. But at the same time, if I'm sacrificing time/money/sanity, the kids have to make choices too, sometimes.
If my DD frequently asked to miss practice for sleepovers/extra school events/wanting to get her nails painted with Bella, etc, I'd probably attempt to steer her to a less intense program (xcel, ymca, etc). JO is a huge investment, and I'm happy to let her do it as long as she is also committed to it. Thankfully, while these are my thoughts, this is pretty much a nonissue in our home.
The unfortunate thing is that there will always be these conflicts, and there will never be enough time to do everything we'd like.
With the number of hours my (somewhat shy/reserved) DD trains at this point, she's probably closer to her teammates than any other friends. She's louder and more outgoing and sillier with them than anywhere else I've ever seen her. She's just most comfortable there and with them. She'd rather be at practice with teammates than most other 'fun' places with people she's less comfortable with. And the more she trains, the closer she is to her teammates, and the further she drifts from "other" friends.