A non occupying co-borrower can not improve the credit score of the borrowers but can help with the income for qualifying. If you are going to do a stated loan then there isn't a need for a non-occupying co-borrower.
When qualifying for a loan with full income documentation becomes a problem because of excessive debt you are allowed to ignore all the payments on the debt you are paying off with a conforming loan (up to $417,000 for a single family residence and higher for a duplex, triplex and four-plex). Jumbo loans require you count the revolving debt payment even if it is being paid off, sub-prime loans do not count any of the payment on the debt that is being paid off even if it is a jumbo loan.
Lenders have different qualifications, as a rule, for different loans. If you are looking for a 30 year loan most lenders will allow 40% of your gross income to cover all of your monthly payments, including the property tax and insurance on your house. With better credit you can generally get to 50% of your gross income for quallifying and up to 55% on a sub-prime
loan. Realize, of course, that sub-prime loans have higher rates and costs that covers the risk of higher qualifying ratios.
On shorter amortizing loans, generally 10 or 15 year fixes, some lenders will go as high as 70% of your gross income for qualifying. The reason is the risk is mitigated quickly because of the rapid reduction of the loan.
Some of the aforementioned rules may not make sense to you but it makes dollars and cents for the lenders. Lastly, all rules have exceptions and you might run into someone who has accomplished something I have mentioned can't be done. Exceptions happen, but not often enough to plan on them.