It is a simple definition, a scab is a person who accepts work that undermines the union. This can be refusing to join the union, accepting less pay or crossing the picket line and working.

So a restaurant worker not refusing to feed a student off campus would not undermine the shutdown of the on campus cafeteria. But any attempt to recreate the cafeteria by the school, on or off campus, would.

You should do the best you can to do nothing that could undermine the strike. A key is who do you complain about and complain to. If public transit workers are on strike, which makes your commute worse, do you blame the workers or complain to the management.