Read Maty Ezraty's response:
Groin injuries can be slow to heal and they are very susceptible to reinjury. I do not recommend that she stretch this area until it has had more time to heal. Groin pulls respond best if you lay off them and avoid stretches for some time. Unfortunately, they can take years to heal. This particular student may need physical therapy if the injury persists. Let's look at some of the ways that a student can overstretch or misalign the groins in Trikonasana.
One is by overrotating the inner thigh up toward the ceiling. The inner thigh is very hard to access. In Trikonasana, it should lift from the inner knee all the way up to the top of the groin. If the student overrotates the leg, the inner thigh can lift up toward the ceiling. This overrotation can damage the groin by overstretching it. Sometimes teachers will instruct students to do this in an attempt to have the leg rotate properly into the hip socket, but this instruction is dangerous, particularly for students who are flexible and have the ability to overdo. It is important to teach that there must be an end to a rotation.
The femur bone needs to rotate into the socket, but once it is properly rotated, the student should stop rotation and work on lifting all four sides of the thigh up off the knee. When the femur is properly set, then the upper part of the inner thigh should release toward the floor. I know this can seem contradictory, but it is very important.
The leg should rotate just enough so that the second toe, knee, and femur all track in a straight line into the socket. Once that is accomplished, you stop rotating and the leg becomes as it is in Tadasana (Mountain Pose). In Tadasana, the inner and outer leg release back evenly. In Trikonasana, the inner and outer thigh release to the floor. In other words, the leg becomes a Tadasana leg.
The position of the leg in Trikonasana conspires to make this difficult. That is both the risk (if done incorrectly) and the benefit (if done correctly) of this pose.
The other way that students hurt the groins in Trikonasana is by pushing the femur bone forward too far, causing a hardening in the groin. This is often called puffing. It can happen when a student overdoes the instruction to move the buttock forward to align the body. This is more frequent with students who are overly flexible.
Look to see if your student's groins are puffy or hard when she does the pose. It is also possible to create this injury in other external rotation standing poses, such as Utthita Parsvakonasana (Extended Side Angle Pose) and Virabhadrasana II (Warrior II Pose). Again, once the leg has rotated and is set in the socket, there should be no more rotation. The inner and outer leg release evenly to the floor. The inner thigh is always soft and not lifted to the front of the leg.
Finally, remember that fear is not always misguided. Your student's body is sending her a message, and she is tuning in. Until she understands how to work correctly or effectively, and has confidence in that understanding, she will not be able to accept your help. I would highly recommend that you respect her fear.
Maty Ezraty has been teaching and practicing yoga since 1985, and she founded the Yoga Works schools in Santa Monica, California. Since the sale of the school in 2003, she has lived in Hawaii with her husband, Chuck Miller. Both senior Ashtanga teachers, they lead workshops, teacher trainings, and retreats worldwide.