Depending on the situation, there are a number of ways to approach this.
Is the space “right next door” a vacant building or undeveloped property? If it is a vacant building, will the existing conditions allow for the conversion into a new theater without significant structural alterations?
Also, will developing the adjacent space, whether as new construction or conversion, result in auditorium sizes that satisfy your operational requirements? What if the largest of the 4-6 auditoriums ends up having a significantly less seat count than the existing single screen theater? Will this be a problem?
Lastly, if the existing theater and adjacent space are connected, your friend can approach the project as an upgrade and expansion of an existing theater.
With any of the scenarios, your friend would benefit by hiring an experienced movie theater architect to develop preliminary layouts with alternatives on how to address the issues.