Hi Emre -
The behavior your customer discovered is not documented and not necessarily designed "into" the product. It's there, though - and we hope users are responsible in terms of not abusing it.
That being said, there is no documented "maximum number of concurrent connections for a single username". It's not something we test because we don't expect the same licensed user to log on three or four times on different machines
This behavior (since it's not documented) could change at any time, as well - so your customer shouldn't assume it'll always work the way it happens to now.
I know I didn't answer your question, but I hope I gave you some talking points you can use when you discuss this with your customer?