That's good. Get the letter and I hope the CO considers it. I just so happened to be checking the refusal rates by reason, and non-immigrant visas being refused for public charge was rare (~1% of the time), and almost always overcome after a 221g (~90% of the time).
As noted above, performance is not considered. The letter from a former employer won't have any impact at all on your case.
If you are unemployed and do not have suitable assets, then the CO may deny the visa or issue a 221g on the basis that the beneficiary may become a public charge in the US. I say "may" because sometimes COs don't ask for the form, albeit I think it's commonly requested by Swedish COs. Either way, the beneficiary should be ready to present the forms and supporting documentation just in case.
A joint sponsor for the I-134 may or may not be accepted at the CO's discretion. That said, a quick search on VJ implies they normally accept a joint sponsor for the I-134. They will need to provide a completed I-134 along with supporting documentation in addition to yours.
Getting a job may help. COs generally want to see a stable income, so a brand new job shortly before the interview may be treated with some skepticism. Again, the CO must determine that the beneficiary will not become a public charge, so they will look at the totality of the circumstances.
