It's hard to say from the available info--"it was given to him because they weren't able to see H"--what did she agree to, S gets it during XH's custody time, or S gets it until he sees XH again? Did he or she set limits on who talks to S? If the situation will only last a few more days, I'd be tempted to let it go as a unique situation, but ensure S doesn't have a phone again for 3-5yrs so this isn't an issue again.
My XW and I get along swell, but our custody terms don't grant her access to the kids whenever she likes. The kids are allowed to access either of us whenever they like.
