IANALE, but I think you can only be pardoned if you've been convicted of something. If you've not even been officially charged (let alone, convicted), what will Obama pardon him for?
Most notably, President Ford pardoned President Nixon for basically anything he might have done as opposed to specific charges.
Slate Article on the Subject: http://www.slate.com/articles/news_and_politics/explainer/20...
Preemptive of any investigation/indictment/etc. Not, as I understand it, in advance of the illegal action itself. (Not a distinction relevant here, of course, just clarifying)
Interview: http://www.spiegel.de/international/world/spiegel-interview-...
The only constitutional restrictions on presidential pardon are
1) He can only pardon crimes against the federal government. That means a presidential pardon won't get you out of a state rap, and it also means he can't use the pardon to affect civil cases.
2) His pardons can't affect impeachment cases. Most notably, he can't pardon himself to avoid impeachment.
Most legal scholars say the president also probably couldn't get away with issuing pardons for acts that haven't occurred yet, since that amounts to a change in the law and encroaches on Congress's powers.