Strictly speaking, a lot depends on what is stated in the tenancy agreement, with regard terms on sub letting, peaceful stay, forfeiture of deposit, etc.
Assume the tenancy agreement is a standard one, all the above terms should be in. The fact that the landlord wants to forfeit the one month's deposit means that the tenant has already violated the terms with regard to sub letting and this in turn trigger the right for landlord to terminate the agreement and ask the tenant to move out immediately.
Still hesitate and say tenant is siah lan and refused to move means landlord himself is not sure of his own position. Assuming he now has the right to ask the tenant to vacate the premises, just gather some relatives or friends and then go to the unit and ask tenant to scram immediately. If tenant refused, just call the police and evict the tenant. Why still hesitate???
Now that things are not well between the parties, why take the risk of waiting one more month? Never know what tenant will be up to in the meantime and at end of one month, maybe see the premises damaged or incur some ah long business and landlord will have troubles for long, long. At that time, even one month's deposit will not cover for the damages.
Don't forget, the fact that the premises is one that sees violation of sub letting rules, the landlord is ultimately the one to face the music if HDB or some other authority comes down hard on such violation.
If want to forfeit deposit, yet don't know what to do next, don't know own rights, is just plain foolish. Create some more noise and wait for tenant to strike back is even worse!!!