This is a visa case, (I presume since it's posted in a visa forum) so USCIS won't see or deal with the affidavit of support. You are correct as to recommending a tax return transcript but incorrect as to how to complete the affidavit. The total income from the three tax returns gets entered in the tax section, even if it's combined income. This is very straight forward. What you quoted above is about what to submit, not how to complete the form. If this person is self employed, it's a bit more complicated, but assuming they are employed, they will state only their own current income where they are asked what their own current income is "that they are using to qualify....". If the person is self employed, say so, and I'll provide the necessary information.
NVC and Consular Officers deal with affidavits of support in visa cases, not USCIS.
I just want to add, that "asking for a friend" noble as it is, is a bad idea overall. There is no "solution" to this situation. No timeline is given. If the "friend" hasn't filed the petition yet, then they'll file another tax return as married filing separately before the affidavit of support is even needed.