You could embed your actual password inside some random string and them tell him to type that random string, which you could make as
complicated and as long as you want.
For e.g. if your password is abc123 then you could tell him to type, lets say this - gibarisbhc3$%&1st2u5f3.
It really doesn't matter what random string you choose to say, so long as it contains all the characters in your actual password appearing in that same order.
Now its just a matter of making him delete the unwanted characters from that random string such that only your password remains in the password field before he presses the enter key to log in to the machine.
In the e.g. above there are 16 unwanted characters that needs to be removed and lets say we choose the method in which we will remove them starting from the rightmost character. You could also adopt some other complex methods (like for e.g. first odd ones and then even ones) of deleting to make things more complicated, in case your friend has a nearly eidetic memory.
Remember that the cursor would be at the end of the last character before the deletion process starts. I'm saying this because you could actually add more complication by not starting from the end (for instance just tell him to press the Home button which would take the cursor to the start).
OK so the first unwanted character that needs to be deleted in our adopted method of deletion would be the last f.
This can be done by telling him to press the left arrow once and then hit backspace and so on and so forth for the remaining unwanted characters.
So it would be nearly impossible for someone to figure out the actual password, assuming he is not writing the password and is simply typing it as you say it on the phone.
P.S. You could also make use of the Insert key during the deletion process to add one more layer of complication.