Or he can add a field where the recipient add their emails "about" (would you like to hearing from us), So if they click on yes then it would be comfortable. Just saying because in my mind sending them even an approval request on emails to provide promotional content! It will make them uncomfortable about it as they would think they are being spay!
Either I did not explain it correctly, or you did not understand me. There is never an "about" section in an email to those on an SOI/DOI list. Emails going to those are already opted in to receive emails. In the emails we send to our loyal subscribers, they typically have content that is article based, product based, or offer based. What I do when I have any, or all, of those (article/product/funnel) available on (or in) another subject, site, or funnel in a related niche that I would like the subscriber to consider is to place a teaser about it (them) at the bottom of the email with a link or links for the subscriber to visit. I have a small and dismissible floating "Update Your Subscription" button on the far right of the page. This system is proven, I have used this for more than eight years now. Not one complaint, not one conversion loss.