No, it's a secret.
Here's Bushnells site, but for some reason it quit working a few minutes ago. I was there looking at there manuals page and the whole site just froze.
Bushnell - Hunting
Optics Planet has a 28mm listed for about $90, not sure which model he has though. It would help narrow things down a bit. I should also say that I've never had much luck dealing with Optics Planet, they don't have the best inventory system. Everything I tried to order through them was back-ordered and I wasn't made aware of it until after my order was placed.
Bushnell 1x28mm Trophy Red Dot Sight 730134 FREE S&H 73-0134. Bushnell Trophy Red Dot Sights, Bushnell Red Dot Sights.
Here's a 30mm for less than $70 at Natchez.
Bushnell Trophy 30mm Red Dot Sight 6 MOA Reticle Matte w/ Weaver Rings - Natchez Shooters Supplies
Here's another 30mm at Pyramid Air, I've bought a few scopes and rings from them.
Bushnell Trophy 1x28 Red Dot Sight, 30mm Tube, 6 MOA, Weaver Rings. Dot sights.
One from Eabco.
Bushnell Red Dot 4-Reticle Electronic Sight
...and one from Midway USA.
Trophy Red Dot Sight 30mm Tube 1x 4-Pattern Reticle (3 MOA Dot 10 MOA Dot Crosshair Circle 3 MOA Dot) Weaver-Style Rings Matte
BSA also has several to choose from.
This one is $40 at their site, probably cheaper elsewhere.
RD30 Illuminated Sight | BSA Optics
Illuminated Sights - Sights for rifles, pistols bow hunting | BSA Optics
You may have to purchase a Weaver or Picatinny rail for her Marlin rifle to use the Red Dot sights listed above. I don't think the two piece Weaver mount set-up will work and you may even have trouble getting the 3/8" dovetail Red Dot sights to work with her rifle being a bolt action. The gap (for ejecting spent brass or loading by hand) between the front and rear of the receiver might be an issue depending on which sight you choose.
Diversified Innovative Products - Product Detail - Rimfire Bolt Action Scope Mount - $37.97