Q: What should I tell a customer (or supplier) when my colleague fails to show up for a meeting?
A: Obviously you need to apologise. You should try to call your colleague to find out what is going on and let the customer / supplier know why your colleague has failed to show up,
If you can proceed with the meeting without your colleague, you should do so. But if you need to rearrange the meeting, make sure that the customer / supplier is inconvenienced as little as possible.