Is there a special Wasabi paste/powder I should use when I'm making Wasabi ice-cream?
To my knowledge wasabi paste is just wasabi powder that has had the water added to it by the factory.  So if you are looking at paste vs powder I would go powder.  The question to me would be to make it a paste before you add it to your ice cream base or to add the powder directly to the ice cream base.  I would venture to say you should be just fine adding the powder directly to the ice cream base.

Now, for the ugly truth.  Pretty much all you wasabi powder sold in the US or Japan for that mater isn't really wasabi(or contains very little wasabi).  It is horseradish, that is colored green and has some sort of mustard or turmeric added.  And really not what I would want to try in an ice cream.  Plus the flavor of the pastes are not like fresh (even with horseradish).

If I were doing an ice cream, I would want to stay away from the powder all together. I would visit my local specialty food market or online specialty market and see if I could source real wasabi.  But cost might be prohibitive for you as it is very expensive.  Next the good news is, there are some powders out there that are actually dried ground wasabi powder such as this one at "The Spice House".  It was most likely something like this that your friend used if she did use a powder.

Anyway, I hope this is helpful to you.  And good luck.