That hard line is notorious for rusting through, a friend near me runs a shop that specializes in Toyotas and he says they replace them often. It's a poor design, there's a bend that creates a low spot on the hard life, and water just sits there. I spent most of last week under my '15 DC replacing rusty bolts and scraping/ painting rust. My hard life is very crusty and feels paper thin on that spot, so I ordered a new one, wool replace it when it arrives. $250 for the past, ouch. Not sure what I'm gonna do to keep it protected so it doesn't happen again.
To answer your question, if he keeps it topped of, it should be OK. The pump will howl and groan if it gets air in it, not good for it. But the risk is if the line suddenly separates, it will pump out all the fluid in seconds. So he needs to get it replaced asap.