Hi Praveen
I would not say that they need an extra license for the pure activation of web services as this functionality is already part of the business suite, but they need to know that, as far as I can imagine your infrastructure, all users logged in the portal (and SSO to backend) will be charged (sometimes customers make agreement with sap considering this type of users "light" ...)
The licensing mechanism is always been not 100% clear so, keep always in mind to recommend your customer a double check with their sap point of contact
Here another similar thread NetWeaver License for Web Services?
Hope this helps
cheers
a