Hi, I'm new on the forum, but I have some information on this subject that might be useful.
I used the HBP program when I bought my house in 2014. The extra $$ tipped my down payment over 20% so I did not have to purchase high ratio mortgage insurance. I'm more than half way through paying back the HBP with minimum amounts each year. It was a good decision.
The 90-day rule is imposed by the government, not the bank, and the bank has no choice but to comply.
However, it's still your SO's money, and he/she/they is/are entitled to withdraw funds from the RRSP account even if it's less than 90 days. The drawdown will unshelter the money immediately and it automatically becomes taxable income. There is another part to the rule that obliges the bank to send income tax to the government as part of the withdrawal of the funds from the RSP. The bank sends the income tax fee the government as part of the transaction. The tax rates are here --
https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/rrsps-related-plans/making-withdrawals/tax-rates-on-withdrawals.html-- a minimum 10% on 5K to 15K, and goes up from there. You'd need to allow for (add) this tax payment in whatever the drawdown amount your SO would be looking for.
The income tax payment is kind of ugh, but depending on the amount you'd save on the mortgage insurance, it might be worth it. You'd also want to ponder lost RSP interest income over the 15-year repayment period.