Your son is legally an adult. He is renting a home. He is making some choices that you disagree with (such as his choice of girlfriend, loaning money with no hope of having it repaid, hiding a relationship status from family and friends). That basically sums it up.
So, the problem is that you really only have control over one factor in this puzzle, and that is, you are the legal landlord of a rented condo. There is a lease which gives the tenant certain rights. You have every right to make a landlord's decision about rent amounts, and not renewing leases, and to seek new tenants or not rent it at all.
You can hope that your son will listen to you, and that he'll make better choices in life, and that he'll listen to his friends, but 23 year old boys often don't make the best choices. The reason your son allows this girl to be in his life may never be understandable. Perhaps he thinks he can help her, perhaps she has appealed to his sense of being the hero provider, maybe she's just cute and sexy and he's not thinking about anything beyond that. Maybe he thinks he'll never find a girl who's educated, independent, employed, and responsible and he's given up trying to improve his life. You'll probably never understand her reasons for latching on to him.
But you really can't kick a kid out of a lease because you don't like his relationship arrangements. Not paying rent, damaging the contents of the unit, or not abiding by the lease provisions are reasons to evict a tenant. We have a house that we rent, and the tenants are just weird. The wife is making some horrible choices, career-wise and in other areas. However, they take good care of the property and never fail to pay their rent. The restrictions outlined in the lease are being followed to the letter, so, live and let live. We only know them in an extremely superficial way, having met them briefly just a couple of times, so we have no influence over their personal lives. We just ask that they follow the lease.
Your choices, as I see them, are to draw up a new lease with a fair rental price, which your son will probably not be able to afford, and he'll move out or get evicted. Or you can study the lease and see if it permits the landlord to simply not renew the lease (be sure you're following the letter of the law). Or you can continue to rent to your son, and hope he grows up pretty quickly.
But whichever way you choose to proceed, please don't make this about the girl. You shouldn't say "your girlfriend is stupid so we're not going to rent to you". That doesn't help your son learn adult behavior. Make this as business-like as though you were not related to him and simply charging a fair rent or terminating the lease.
You say you wanted to help your son get on his feet, but allowing an adult to live in a condo for more than a year at half the reasonable rent is doing anything except helping him get on his feet. It's more than about money. If you want him to get on his feet, make him make adult choices, like applying for an apartment or room somewhere, and not being accountable to mommy and daddy anymore. If you must, help him with the down payment for that room. Tell him you're always willing to listen, to advise, to counsel, to help with business decisions, but you're not going to carry him on your backs anymore. And stick to non-emotional discussions. Don't talk to him about the girl. Talk about not loaning money, about paying bills, about living an adult life.