In a mixed level class, I encourage students who are challenged with Adho Mukha Svanasana (downward facing dog) to hold cat pose instead. If the class is practicing full Surya Namaskara’s (sun salutations), modifiers can practice cat/cow instead and still stay with the pace of the breath and class.
One thing that I like to do with individuals whose wrists are tender (regardless of bodyweight) is do the movement against a wall. Starting with hands up on the wall as far as comfortable, then walk away from the wall while still touching it, “leading” with the tailbone. Chairs and steps are great if you have them in the room. For the rooms I’m in, a wall is easier to access.
Until the person in question has gained an appropriate level of fitness why not try breaking down the movement into segments.
The downward dog is combined wrist extension, shoulder flexion to approximately 170-190 degrees, hip flexion to 90ish degrees and full dorsi flexion.
Have the person work on these isolated positions and integrate them all once they are prepared to tolerate the downward dog position.