Toby Cryns replied to the topic Customizing Buddypress Activation Emails (and still get activation key) in the forum How-to & Troubleshooting 8 years, 4 months ago
@gentlemanhog – If it works, then it doesn’t matter where it is. 🙂
The conventional wisdom is to keep things out of functions.php unless they are theme-specific. In your case, I would say this function belongs someplace other than the theme’s functions.php file.
@gentlemanhog Not recently active