The Rusticani are supposed to be able to put any supernatural effect they can cast themselves into devices, so that others can cast it in their stead. So, for these devices, it is as if the rustic magus is casting it for the wielder, and that's why these formulas use the character's Casting Total instead of a Lab Total. This means that unfortunately, I think your player is right that they are limited to using their formulaic spells when using this Virtue to craft items, or else their Casting Totals divided by 5 with no die roll for spontaneous effects.
They can still enchant devices, though, just like any other magus, and they get to use Design and Inscription bonuses when they do this, which is a nice perk. If they're not crafting one of their inherent powers into an object, it still takes them a whole season to do it, but at least they can invent whatever they wish. Enchanted devices are where rustic magi should really shine. Unlike Verditii, rusticani should go for quantity over quality, and I'd imagine they typically have lots and lots of magic items on them at all times. And as a storyguide, I'm thinking I'd allow a rustic magus character to use an enchanted item that he made like a spell he can cast, as far as crafting more of them goes.
Say our rustic magus makes a magic sword, with a power that sets it on fire, like the Level 15 CrIg spell. As for any magus, this takes him some vis and a season to invent it as a lesser device, since he doesn't know the spell. After that season, as he has his finished his magic sword, he can craft more charged items to do the same thing. He makes a Craft: Weaponsmith roll with a base level of 5, since activating the power doesn't normally require a die roll, and he gets a 10, which means he can make two of them. I'd guess this takes him somewhere around a month (I don't have City and Guild in front of me).
Taking other Virtues that boost his magic is always prudent, I agree. I'd certainly suggest he take a magical focus. However, I'm not sure that Virtues that specifically help with learning new spells would be as worthwhile as Virtues that boost his casting totals or help him increase his Arts and Abilities. Ultimately, he's going to want to spend his downtime crafting items more than inventing spells, I think. Interesting Supernatural Abilities (for more stuff to craft) and things that make him better at crafting are going to go a lot further, I think.