Well if its that's the case, then just be a darling and give
that poor man a hand. That's what friends are for isn't it? Your
father is his friend's savior for having a son like you who's the
expert in that area!
On another hand, maybe you can use this case as a lead-in to
explain to your father that you would appreciate he get his
friend's contact and pass it to you to call instead of giving away
yours, since you don't seem to feel comfortable about it (from your
first post)
His friend will have his questions answered, you have the control
of when to call, and you keep your privacy. Win-win! 
If this is a first case, probably your dad didn't know about your
preference. Communicate to him and let him see the reason.
I did that with my mom too, and from then on she won't give my
mobile away or agree to do something for her friends on my behalf
without asking me first. 