UCMA: Chat with users not in AD

Question

a customer wants enable a chat/instant messenger for his application webside. He is using Lync Server internally to Chat in-house. Now, he requires the following: A external user (which will not be an AD user) logs into the webside is able to chat with a person inside the company. The internal user will receive those messages via his lync client.

What's the best way to achieve this? i thought about bot that delegates messages from the webside to the lync server that does the rest. But how can i send a message as an external user?