That's a tough one. They could help each other out and support one another in their recovery. On the other hand if one of them falls the other may follow, especially early on in recovery.
Also, is he or she in a treatment center for the detox? If not, encourage them to seek medical attention if it gets too bad. Withdrawals can be very dangerous.
Might be best if they did do this separatly for a few weeks. Could she stay with you for a few weeks?
Remind them that they can get clean if they truly want it.
Best wishes and God bless.