One of our office managers is leaving in 2 1/2 weeks, although she had given us a months notice. My HR manager told me to inform her boss that it would be illegal for us to bring in a replacement before she leaves. Is that true???

One of our office managers is leaving in 2 1/2 weeks, although she had given us a months notice. My HR manager told me to inform her boss that it would be illegal for us to bring in a replacement before she leaves. Is that true???