Open Steam and sign in
Go to your Friends list
Select the friend whose wishlist you want to view
Open their profile
Click the Games tab
Look for Wishlist if they have made it visible
If you do not see Wishlist, the friend has hidden it or changed privacy settings
You can also open their profile from chat or the friends list and check the same tabs
If their wishlist is public, you may be able to view it directly from their profile page