You have correctly analyzed the situation regarding the difference between a static IP address and a Unique IP address. The Unique IP is subject to change, and you will need to change it in the client’s nameserver records if they maintain the domains records there. This can be a pain, indeed.
Unfortunately, DreamHost does not, at this time, offer Static IP addresses.
Given your description of the client’s concerns, one workaround for this is to just use DreamHost’s namesevers with custom MX records for email set to the client’s static IP address. As those records are static, this seems the way to most easily and conveniently deal with this if you want to host the website here, but the email on their IP address.