James Huff, surely that is incorrect advice that you have given? A sub folder would be best for the OP. A new domain is ideal, however, it might not be possible as the second site is indeed connected to the first.
It makes sense to use a subdomain if you are:
Language Barrier
Setting up a site that caters to different languages. Diffrent versions of your site that cater to different countries can be put in subdomains such as: jp.example.com, in.example.com, ph.example.com. Putting these in a subfolder would make your site seem huge but have the same content – just in different languages.
Franchises
Sometimes you have people who bought a franchise from you and you want to keep them (or they want to stay) on the same SLD. In this case, you may not have complete control on whatever they are going to do with their site so you’re better off giving them their own subdomain rather than putting them in a subfolder and risking your entire site when they do something stupid with their SEO.
Conflicting URLs
There will be times when your SLD site is so big and you find yourself in danger of overlapping the URLs. If that’s the case, use a subdomain. That way your URLs will never conflict with any of your SLD’s URLs.
Tips for Keeps: The unbeatable advantage of having your subsite as a subfolder is that it gets all the link equity and other SEO factors that your SLD receives.
https://seo-hacker.com/subdomain-subfolder-affects-seo/