If their strap is twisted, untwist it. Always carry extra feminine products, and extra hair Ties. And if she’s being harassed, you just link up and walk with her, no questions asked.
Also, my daughters have a literal code... if they ever text me the secret code, I will drop everything I’m doing and call them immediately, make up a major crisis and demand to come get them immediately. Make it 100% my fault, but give them an immediate out to any situation they don’t feel able to simply walk away from.