Danielle’s recommendation is right on. A blood clot is potentially life-threatening, and the condition that lead to it may still be there. She also may be on medication, and some of those medication can have side effects.
Definitely get a physician’s release. This may even give you an opportunity for establish a relationship with a doctor. Doctors often look for trainers to refer to as long as they are confident that the trainer defers to the physician. if in doubt.