How to pronounce Zengyang?
Well this is a bit of a wild guess - but we don't really have much to work with do we now...

for the first part of your friends name you can take a look here:
曾 and click the audio button next to [zēng]
(this most likely is the correct pronunciation for the first part of your friends name)

Yang is most likely a second-tone (again best guess)
so you can have a look here:
阳 and click the audio button next to [yáng]

This is the best I can do you for.