One thing you really can’t fix in the mix too easily is strain. Even if you pitch the vocal well, there is a characteristic in the voice that comes through as strain. Singing flat does sound like strain so if it’s pitched correctly it will sound slightly better, but strain can damage the tone of the vocal.
First you need to identify what is causing the singer to strain. Strain can come from not warming the voice up with exercises, being dehydrated, not hearing the voice loud enough in the headphones, being tired physically, having a hoarse voice from a recent performance, or singing notes that are above or below the vocal range that the singer is accustomed to singing. Even though it can seem overwhelming, as a producer, we must start experimenting with trying different methods.
I would first bring up a good vocal warm up on YouTube and have the singer exercise their voice. Then I would check the headphone mix and make sure their voice is loud and clear. If there was a recent performance that damaged the voice, then they need rest. Reschedule the vocal session. Lastly I would try pulling the mic out of the vocal booth and have the singer sing in a bigger room. Sometimes the sound of a room around the singer can bring an environment that they are used to. Comfort level is a big factor when recording vocals. No matter what, stay patient and help the singer eliminate the strain!