There's no such limit -- unless he's exceeding his own mailbox limits...
For instance we allow users to have up to 50 MB stored on our Exchange Servers. When their mailbox exceeds the limit, they cannot send or receive any more until they movesome down into their PST files. Essentailly they have to make space on the server (in their account) before they can send more.
If your client is trying to send a relatively large file to hundreds of users (1 MB file to 400 users-- we're talkingabout 400 MB at once -- which out servers would NOT allow), he may be encountering his own server limitations. Look at storage space on the server and look to see if there are any limits. Check Event Viewer for warnings or errors.
There are no limits outside of what is configured on the server and how much free space is on there.
--- Check with his ISP if he does not have a Exchange Server on his LAN/WAN.