In essence, putting her in a custodial setting in which the lawyer has an interest, be it a halfway house, rehab or whatever you might call it, is making the lawyer her bail-bondsman.  The lawyer is vouching for her being in a particular place at a particular time and (insofar as required by the court's judgment) doing particular things.  That means the lawyer is either her jailer (if she's in custody), her guardian/conservator (if she's not up to managing her own affairs),  or her bail-bondsman (guaranteeing her appearance).  The problem with each of those is that, to a greater or lesser degree, these positions and duties wind up putting the lawyer in a position of having to violate his duties to his client in order to fulfill his duties to the court which appointed him.

To put it in perspective, suppose the lawyer is convinced there are good grounds for the jail sentence being illegal or otherwise susceptible to reversal on appeal.  If the lawyer keeps her in custody in his half-way house, he is conceding the argument that her sentence is illegal or reversible.  Not only that, but the lawyer is not doing the jailing for free. Someone is paying him for the privilege of holding his own client in custody, and he (like all private custodial facilities) is making some margin of profit on that payment.  So he stands to profit from holding his client in custody which he believes as a matter of law might be improper, and is getting paid by the client for fighting for the client to get that very jail time eliminated, reversed or at least reduced.  The lawyer winds up serving two (his profit interest and his client's interests) or three (profit, client and duties to the court) masters whose interests are in direct contradiction to each other.  This is a situation far worse than the ordinary representing-two-clients-at-the-same-time or in-succession kinds of conflict of interest.

Something similar obtains if he's supposed to act, in essence, as her bail-bondsman.  A bail is supposed to guarantee the presence of the defendant by making the defendant liable to the court for a sum of money if the defendant skips.  The threat of lots of money going out is supposed to give the skip-prone defendant pause.  A bail-bondsman is, when all is said and done, an insurance company.  He writes a form of insurance - that the defendant will appear when and where commanded by the Court - in return for which the defendant pays him a premium.  The bondsman's interest (like any insurance company) is in not paying out the policy and he effects that interest by finding and bringing the recalcitrant defendant to court by any means necessary.   The lawyer who might act as bondsman winds up being in the position of being a guarantor of his client's behavior and giving up his client to the court and doing anything necessary to effect that, regardless of the client's interests, all in favor of the lawyer's pecuniary interests.  Again, ethically treacherous.