My machine with Crystal on is unavailable right now, but from memory there is a function you can use which returns a number representing the day of the week - I think it is Day(Today) - you can use the Crystal Help to find it. You then get something like 1=Sunday, 2=Monday, etc., but please try it out to confirm what number is which day because I can't remember.
Now to get last Friday all you will need is a formula that says something like:
Today - Day(Today) - 1
I put 1 in the above on the assumption that my memory that Sunday = 1, Monday = 2 and so on is right.
Sorry, but I've got to dash out now. If the above doesn't work for you then please post back and I'll retrieve my machine with Crystal on.