This is very straightforward. Your friend's advice is wrong.
Your wife used her Cambridge KET A2 pass as part of her successful application for her spouse settlement visa/entry clearance and so she can use this same KET A2 pass for her FLR application even though the Cambridge KET exam is not on the current secure English language test (SELT) list.
Paragraph 32D of the Home Office Immigration Rules Appendix FM-SE: family members specified evidence (14th July 2016) applies:
"32D. If an applicant applying for limited leave to enter or remain under Part 8 or Appendix FM submits an English language test certificate or result and the Home Office has already accepted it as part of a successful previous partner or parent application (but not where the application was refused, even if on grounds other than the English language requirement), the decision-maker may accept that certificate or result as valid if it is:
(a) from a provider which is no longer approved, or
(b) from a provider who remains approved but the test the applicant has taken with that provider is no longer approved, or
(c) from a test centre which is no longer approved, or
(d) past its validity date (if a validity date is required under Appendix O), provided that when the subsequent application is made:
(i) the applicant has had continuous leave (disregarding any period of overstaying of no more than 28 days) as a partner or parent since the Home Office accepted the test certificate as valid; and
(ii) the award to the applicant does not fall within the circumstances set out in paragraph 32B of this Appendix."
For FLR(M) applications submitted from October 2016, the minimum level accepted for English speaking and listening rises from level A1 to level A2. Your wife's level A2 KET means that she already meets this change.
Hope this helps.