If the wife does not have a share of the HDB, why would she contribute cash to the hdb? He's not thinking straight. As for CPF, the law does not even allow his wife to draw from her cpf, if she is just an occupier, not owner.
It's actually a pretty common scenario when the spouse is not a PR or citizen. The mortgage is too expensive for one person to tank alone, and BTO is not an option because it's not allowed when the other party isn't PR/citizen, and going for flexi unit under singles scheme doesn't make sense if the couple wants kids. Build finish, kid is 5 years old. Wait until can sell and upgrade, kid is 10 years old. One couple live in the same room as their 10 year old child/children does't make sense.
This is pretty much the arrangement i currently have at the moment. Her options are to either contribute the cash, or the marriage cannot happen because we cannot afford to get our own place to live.