So, should she have stocked parts for 3 months, 6 months, a year, two years worth of production?
The, up until recently, stable global supply chain, means people don’t keep big inventories of parts. Real time logistics and shipping has taken care of the need to sit on large inventories.
But, she should be able to plan for this, right?
To answer your question, "she should be able to plan for this, right?". Yes. I've worked in manufacturing for 25 years and we live and die on parts availability and when pricing is expected to go up or availability is uncertain, etc. we would be more aggressive about buying critical parts and stocking them to stay ahead. It's just risk management really and if her business is ISO 9001 certified (not sure if it is) then she should be fully aware of what risks are on the horizon as they would talk about it regularly.
"On time" manufacturing is great until it isn't. You still need to plan around risk.