Aw, that's too bad.

A 2.6% HDB loan was/is cheap money.
You meant a 75% loan, correct? That's the maximum bank mortgage available, and that's also assuming there's no cash over valuation complication. The 80% figure is for HDB loans, and I can't see how he qualifies for another HDB loan in these circumstances (second home).
If ABSD is S$140K then that implies a home purchase price of S$700K (a VERY small shoebox condo). 75% financing is $525K. Assuming he celebrates his 50th birthday tomorrow he can probably get a 15 year loan at 3.0% fixed for 3 years. (You can only get a mortgage to age 65 for a new mortgage. Upon refinancing it's usually possible to stretch that to age 70.) That would mean a monthly payment of $3,626 just for the mortgage, not including property tax, maintenance fees, income tax (if rented out), and other costs. There's a collection of risks: interest rate risks, investment risks (home prices could fall), rental rate risks, currency risks (the Singapore dollar could lose real purchasing power faster than expected), and tax/regulatory risks (the government is likely to hike real property-related taxes first and further). And the $140K in ABSD is a pure sales charge. The home has to appreciate to $840K just to break even on a nominal basis, not including transaction and other sales-related charges such as ordinary BSD. ABSD alone eats up the first 20% of price appreciation, which is a lot. That's
many years of price appreciation even in an appreciating real estate market, which isn't a given.
As others have pointed out there's no problem with SERS if the private property(ies) was(were) acquired prior to the SERS announcement.
There's a great deal of risk in a real property purchase, especially when starting off an additional 20% in the hole (ABSD). A well diversified, low cost stock index fund isn't THAT risky. In particular, no bank is going to come after you (and your assets) if you can't pay the mortgage.
T-bills, Singapore Savings Bonds, money market funds, fixed deposits, and endowment plans are all comparatively low yielding vehicles. A low cost investment grade bond index fund is highly likely to be a bit higher yielding (over the long-term at least) but still extremely conservative. (Too conservative on its own, I'd argue.) CPF MediSave, Special, and Retirement Accounts all currently earn 4.08% interest (4.05% in 2Q2024), sometimes with tax relief when you contribute.