Thank you for your thoughtful answers, everyone. Since posting this question, she has continued to use the climber about once a week, as part of her varied program, and heartrate usually peaks at about 150. She has not had any discomfort or contraindications. She is not on any medication, and has been active since age 30. Perhaps her regular use of the subway stairs has been a benefit, too!
Some further thoughts, as I was somewhat taken aback by the comment that 70 is too old to use a climber. In my opinion, being able to step up is a basic ADL skill. I’ve seen people move from a multi-story house to a retirement community, start taking the elevator to other parts of the building, and lose the ability to use the stairs. Even getting into and out of a car, or onto the curb, requires stepping up.