Hi Kristi , in our tests, @ mentions on iPhone are working correctly. I've noticed occasionally that if the @ is typed at the start of the post it can sometimes not bring up the mention list. If this happens try typing a space or other character first before the @ symbol. If she continues to have issues, let us know. Thanks!
We couldn't identify any issues at this time. However your site has code customizations added by your developers. It is possible one of these is failing and causing the issue. Please have them check. They should be able to determine where the error is coming from. We have not been able to reproduce this on our end with a standard configuration, which indicates that it may be one of your code customizations.