I don't know how helpful this will be but I'll chime in anyway. I can't speak to this particular climb, but I live in Tucson and the climb up our local mountain, Mt. Lemmon, goes from 2400 feet to 8200 feet above sea level over a distance of about 25-30 miles. (depending on where you start). Also, it does have a section at the top where it goes down and up again, so it comes out to about 7000 feet of climbing over the 60 miles or so. The overall gradient on Mount Lemmon is about 5%, but it varies from 3-4 to more than 7%. I've never ridden the last bit to the observatory, which is much steeper and goes from about 8500-9000 feet.

Before I rode to the top (well, almost the top), I did gradually longer segments over a period of weeks and that was very helpful.

Since Tucson is already at 2500, the altitude isn't quite a much of a problem as it would be if we were acclimated to sea level.

I agree on layers coming down. We always check the hourly weather forecasts before rides like this, so we don't have any unpleasant surprises that we aren't prepared for.

Personally, I'm really slow, but I get there. I take lots of breaks and rests and even though it's a long way, that keeps me going. So I would put in a big plug for your client to pace herself and to be prepared and comfortable with the idea of taking breaks as needed. And eating regularly!