Free riding requires non-excludability. This good is just non-rivalrous and on exam you should generally not make up further assumptions other than those provided. If a good is non-rivalrous but excludable there is no free riding problem. Any potential free rider can be excluded by the firm.
For example, an empty seat in half empty theater is in essence non-rivalous (if we know for sure the theater is never full). However, theater owner controls access to the theater so there is no possibility for people to freeride since everyone has to buy ticket to enter.