Since you are the sole owner of the HDB, then I assume that you have used your CPF to pay for the flat. Accordingly, the accrued interest is under your CPF Ordinary account.
Upon your demise, your partner will become the sole beneficiary (assume 100% share). Accordingly, she will inherit the HDB if the law allows her to do so. Meanwhile, she does not need to refund the monies withdrawn and its accrued interest to the CPF account.
In similar fashion (after your death), and upon the demise of your partner, her son will inherit the HDB. in the same way, he does not need to refund the monies withdrawn and its accrued interest to the CPF account.
However, take note that if your partner is the sole beneficiary and she passes away before you, your Will will become invalid since there is no other legal beneficiary. Consequently, Intestate Succession Act will dictate the distribution.
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CPF Accrued Interest lives and dies with you.
It will not be passed down through the transfer of your property.