Yes. That is by using the Broadcast Pincode. You can generate a broadcast pincode for each campaign and they're unique to each campaign as well. Your client can send an SMS broadcast immediately by texting to the campaign number and the message should be in this format: "pincode (space) message". (not including the quotes)

For example, if the pincode is HH11, the client's text needs to be "HH11 Hello this is a broadcast". The subscribers of that campaign would receive the message "Hello this is a broadcast".

The Broadcast pincode is located and can be generated in the campaign properties, particularly in the Notification Settings. First, hit the Actions menu of the campaign then click Properties.

Next, open up the Notification Settings.
Then scroll down until you find the Broadcast Pincode field. If there's no code populated in there, just click generate.

Broadcast Pincodes are unique for each campaign so there won't be any mix-ups in the campaign subscribers.

Note: This works only for SMS Broadcast. If you need to send an email broadcast, then you can use the Broadcast Email method.