How does an 18 year old who works at Wendy's come up with the cash to buy his own truck and spend $2K each on rifles?
The truck could be leased or financed. Very possible for a diligent saver.
https://palmettostatearmory.com/ar-15/ar15-guns/rifles.html
You can even buy the uppers and lowers separately and assemble them yourself for even cheaper.
Smith and Wesson M&P Sport goes for a bit more.
Note: not a firearm owner, so maybe there's additional costs involved that I'm not aware of.
Possible he used her credit card(s) to purchase the firearms.