Not necessarily snappy, depending which of the Titans he uses and adjusts accordingly.
The standard HW synthetic guide is actually really, really good. A good and strong two-piece design. If it fits the selected spring properly, no need to look any further.
If he finishes the spring off properly, and with the slip washer, all might be very, very good. I'd still always prefer a top hat at the front end of the spring, giving more support and an extra bearing surface with a polished steel or synthetic washer ahead of it. These days a spring just doesn't seem properly dressed without one!
Although a lightweight, the 99 isn't obliged to be snappy. Indeed, in .22 they can shoot very demurely at close to legal limit power. With a correct set-up and running around 11 it should be extremely nice to shoot.
Enjoy once back; I'm sure you will.