I've seen characters like that before.They're possible, and chances are he either didn't know you had no experience as a SR GM, or counted on it.
As soon as the game became yours, you got veto rights. Should have double-checked everything, and simulated a 1v1 against a Prime Runner of some sorts, to figure out who is the most combat capable. Then check other major areas - 20+ dice is totally possible in a Face, at chargen. If you can't create something that can reliably take them down, one on one, tell them to roll up a new sheet, that this is not the other guy's game, it is yours, and you have your own balance to keep.
Magic is a headache for new GMs. I'm still tripping over it here and there. Against all that Cyber, go the DnD route - Pitfall trap! Remind the players, before and after, that they've attracted the attention of some serious competitors with actual heads on their shoulders. If they've been running against literally anyone, they've probably made an enemy or two. Consider how big the bounty on his head would be, if Ares decided to bring him up on false charges, and consider how the guy would respond to seeing that yacht they were hired to defend during an off-coast voyage, sink. With them on it. If the bounty was high enough, it'd be negligible cost to hire the team and drop them to the drink. Then the party can contend with him, for bringing it down on their heads.
Check his mental scores. Is he Intuitive, or would he not notice the tac-team following his every move? Is he Logical, or can he not connect the dots that his Doc is filling him with poison during the upkeep? Is he Charismatic, or are his Contacts gunning for him? Is his Will strong enough to resist a barrage of drugs and BTL long enough to complete a very boring job, or is he going to make sure no Johnson hires his team again?
If he's not thinking straight, because he had a dumpstat, then take advantage of it.
Oh, and Chummer isn't perfect. It really, just isn't.