All the questions you mentioned would be entirely appropriate, especially how many total, how many per year, and success rate. Honestly, 500 ablations in 7 years isn't a lot. At all. I'm sure he's a fine EP but he isn't who I would choose. Make it 5000 and we're talking.
The generally accepted criteria of success is complete freedom from sustained atrial arrhythmias for one year without the aid of antiarrhythmic drugs. The EP I would choose would be able to claim an 80% or better success rate with paroxysmal afib. If your friend has persistent afib, that's a whole different story and would require only one of the top EPs in the country.
Why is time of the essence? Choosing an EP for an ablation isn't something I would rush into.