If they're lowers never used on anything, they are an "other" and must be transferred to him by a dealer in his state (because they could become a pistol). If they have a butt stock installed, then it's a rifle and can be transferred to him in any state that allows it (which MO does allow it)......but then it's always a rifle. If you're just loaning them for "lawful sporting purposes" then nothing is required.
As far as checking them on the airline, if he doesn't have a lockable hard sided suitcase, then he needs a hardsided lockable box inside of his soft sided luggage. Not sure if frontier allows multiple firearms per bag. Some airlines only allow one. Frontier sucks, but no problems checking a firearm last week for me.
Are these anything special?? Might just be easier to buy a couple on your credit card and have them transferred to him in is home state.