I've refinanced a bunch of times (including through SoFi, Earnest, and Credible) and have found Earnest to be the best for customizing loan terms, in case that's useful to your partner. Earnest also gave me the best rate (by far) on my most recent refinance. But they should just apply everywhere; there's no reason not to and different companies are likely to prioritize different elements of applications (i.e. one company may be more interested in credit score, another may be more interested in income, etc.).
One thing to note is that SoFi is trying to become an all-in-one financial company for people; they now offer checking accounts (with 0.25% interest), mortgages, and brokerage accounts in addition to student loan refinancing. I actually have my primary checking account with them. There may be benefits to having multiple types of accounts; it's worth looking into if they end up giving your partner a good refinancing rate.