Planking is not inherently dangerous during the third trimester of pregnancy.
That said, as mentioned earlier, it may cause pressure on the linea alba. So, if you notice any bulging along the midline of your client’s abdomen, you may wish to instruct them to perform planks on an incline or skip this exercise entirely.
The good news is that you have many other third-trimester-friendly core exercises to choose from, including the suitcase carry or the half-kneeling palloff press.