(details of how this happens don't really matter here I think).
But that's the juiciest part. (wall of text incoming)
No, seriously. Having a description of what is going on can actually help you determine the difficulty. Different approaches to the same problem can have different difficulties entirely.
You could, for example, say the caster is manipulating the lock magnetically, sending out a pulse to know exactly where each piece is and feeling his way through the combination to know how he has to turn the dial. Would be very easy to open a lock that way, and it would leave virtually no trace.
On the other hand, you could rely on pure force to rip the safe apart at the seams. Doable, especially with magic, but extremely taxing nonetheless. And don't even talk about the difficulty of putting it back together without anybody noticing what happened. I'd probably go with 3-5 for opening, if he had some sort of symbolic link (like the glasses of the owner, because they've seen him open it so many times). 10+ for smashing open at least and even more for closing it again. And even with all that, there might be some traces it has been opened regardless of how good he tries to hide it.
Or you could get really creative. I'm assuming he wants to get at some document inside the safe, to see what is written inside and then leave things as they were. In that case, he could sort of make a photocopy spell, if the ink the document is written in has some sort of magnetic or similar in it. Graphite in a pencil would work best, but we'll take what we got. Then he could paper and the same kind of ink (maybe if he gets hold of the printer or the pen it was written with), press his left hand on the safe and let the magic scan for the letters on the document, while his right hand is moved by the magic he pours into it and sort of copies the entire document without the need to actually open the safe.
This last one, I would probably not even let him roll on. If he has the necessary ingredients (ink, pen, paper, all preferably at linked somehow to the ones in the safe), of course. It's just a pretty cool idea to get what he wants, and the difficulty lies in the planning, not the execution. If you don't want someone to fail, don't roll. Let them fail where it matters, when he is trying to get the ingredients for the spell. And don't necessarily make them final. If he can't get to the pen one way, he might be able to come up with a different plan.