Can an i-Mac monitor (2019) be plugged into a MacBook to provide a larger screen?

No. The 2019 iMac does not support Target Display mode natively.

She could use a number of third party alternatives such as LunaDisplay to use the iMac as a display for another Mac though, but those can introduce lag and other issues so are not suitable for video editing or high paced games, but can work for things like Word Processing, email and web browsing pretty well.

https://astropad.com/blog/step-by-step-guide-how-to-use-your-imac-as-a-monitor/

Grant Bennet-Alder wrote:

Read that article with Caution.

The article Phil0124 cited is not an impartial review by a disinterested third party.

It is an advertorial by Astropad, the maker of Luna display.

Even that article acknowledges that

"Like any product, Luna Display does have limitations and it’s not recommended for high-motion content. For example, I wouldn’t recommend using Luna Display for gaming or video editing.

Since Luna Display is squeezing a 5K display through a USB cable, when lots of motion occurs on-screen (like from video playback or moving a window quickly) the screen can temporarily become pixelated before it renders in at full resolution."

Read that article with Caution.

The article Phil0124 cited is not an impartial review by a disinterested third party.

It is an advertorial by Astropad, the maker of Luna display.

What model-year MacBook Pro as source?