Hi @Jimbo2109
The first thing to check is he has the correct message centre number, this is +447785016005, check he has messages set to text and is not sending as chat here: Messaging Features
Most probably the phone has temporarily lost it's connection, sometimes the SIM can be a little behind and needs to be updated on the network. Ask him to complete a manual network scan: settings > connections > mobile networks > network operators > manual > when the scan completes, select any other network, let it reject the request and reselect Vodafone. Follow this will a reboot.
If the problem persists, he can order a replacement SIM for the number here: SIM swap
Further information for his phone here: Device Support
