I agree with SSDR, but for other reasons.
If you look at hacking, and what it accomplishes, some things are beyond the realm of the Runner. The next closest thing to this is hacking ownership... and look at the mess those rules are, and the interval that is required.... Now, try hacking the tightest database on the planet (Sin registry) AND the next tightest database (Government)..... this is not something done in a few rolls.
"Hacking" SINs requires entire criminal organizations that employ more then slicing electrons to make and copy those SINs... (And this goes beyond the meta-game reasons of "breaking economy" and "breaking Social Systems" and whatnot...)
Also, Think about what the player is really asking you here:
"Bob" wants to fuck over "Fred", and hack his FAKE (hence only rating 5, as all REAL SINs technically don't have ratings.*)...
All this does (IF it was possible!) was fuck over the actual person that the SIN is copied from (At rating 5, there is real DNA on that SIN, and it links to someone close to "Fred" DNA wise, but not "Fred" directly).... and burn the fake SIN for "Fred".
"Fred" buys a new fake SIN, all is good.
Really, all he is asking to do is just burn "Fred's" Fake SIN... A whole lot of dice rolling and table time wasting for what could happen from any credstic reader of quality..
And I ask you, is this really a road you want to go down?
*Some people argue that Rating 6 SINs are real SINs... but even Rating 6 SINs will fail a through enough check, if someone really has an interest in looking that closely. But a real SIN can never fail a test; unless its been tampered with (AKA some organization has made a copy of it and is up to no good. A Government, or SIN Issuer has changed it..)