We cover modifications in all of our Yoga teacher training courses. The solution depends on the reason why your student cannot practice these asanas and if these postures will benefit your student. If the reason is a neck problem or high blood pressure, Viparita Karani (Legs-up-the-wall pose) would be a good substitution for both asanas.
If is a matter of modification, you can use a wall for both asanas. In Halasana, a wall behind the head gives your student a variety of heights and depths to work with. To use the wall in Sarvangasana, start is Viparita Karani, bring the legs hip width apart and ask your student to slide the feet down (around 12 inches depending on your student's size) with the soles resting on the wall, bend the knees, and then raise the lower back off the floor by pressing the feet into the wall. This is a modified version of a supported shoulder stand.
You can view modifications and demonstrations at our video archives.
//www.yoga-teacher-training.org/category/videos/