For a UK Visitor visa application details of any previous refusals have to be provided when completing the online application form. UKVI will have a record of the refusal and the reason(s). It’s not therefore necessary to duplicate this by explaining again in a cover or invitation letter.

For a UK Visitor visa the applicant must provide details of any previous refusals when completing the application form. UKVI will have a record of the refusal and the reason(s). It’s not therefore necessary to duplicate this by explaining again in a cover or invitation letter.

Edit following OP clarification: despite having a sponsor the onus always remains on the applicant to prove that they qualify for a visa in their own right. See UK visa refusal on V 4.2 a + c (and sometimes 'e') IMHO therefore, although it’s not essential, providing a brief explanation of your father’s visa refusal in the context of his sponsorship won’t do any harm. This answer regarding the suggested format of a cover letter may help you What details should a good cover letter contain to back up a UK Visit Visa application?