I think you’ve gotten some good advice already about establishing a good cardio base for your deconditioned client. I’d just like to add that she can get some cardio benefit from the strength training itself. If you are working with your client on strength training, you will gradually notice her ability to move from one exercise to the next with better endurance (assuming her weights are appropriately heavy enough to challenge her). You could try alternating upper and lower body exercises to keep the heart rate up–appropriate for her level–and monitor her Rate of Perceived Exertion. As her stamina progresses, you could try adding in some low intensity intervals or even adding some upper/lower body combo movements as intervals.
I would go slow and focus on proper form and RPE–then take it from there.
I agree with everyone else. Keep it simple, build a good cardio base and establish correct form on strength. As she progresses and builds confidence in herself then think about including interval training.
I’m with Karin & Louis. Just get her going with some easy CV exercise & include some basic strength training exercises – focusing on form. When she gets the hang of things after a while, then start mixing things up if you want. At this beginning stage, don’t worry about things like interval training – it’s too much to process. I would just keep it simple for now & let her experience some success – then progress from there. Good luck Kelly.