According to Stephen Hawking's website, his computer was custom-designed for him by Intel, so it's not commercially available.
http://www.hawking.org.uk/home/hindex.html
The best place to enquire about speech sythesis technology would probably via whichever charity specialises in helping people with your friend's disability. If you have difficulty in tracking down a suitable charity try posting again (perhaps in Body & Soul?). I'll be happy to help.
Alternatively, you could try an e-mail to Professor Hawking. Details are here:
http://www.hawking.org.uk/info/cindex.html
(NB: Take care to ensure that your e-mail is in plain text and not HTML).
Or, perhaps Intel could tell you whether Professor Hawking's computer uses commercially-available speech synthesis software:
http://www.intel.com/intel/general.htm?iid=contactus+com pany_email&
Hoping that something here may help you and your friend,
Chris