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There's a "sticky" post on this very forum that's a bit old but not outdated by much. Start there.
Business credit is VERY EASY to build, and also very easy to screw up. It just takes time, some patience, and a good meticulous use of paying on time or preferably well ahead of time.
If she has a business for 4 years without an EIN, she's throwing money away totally and completely. The first thing she should do is IMMEDIATELY find out if she can incorporate, if there is any backdating she can do on the income she earned last year (or prior years), and become official. Sole proprietorship is the worst tax situation to be in. S-corp makes sense even for many small one person shops.