1. your wife must return to DOHA before completing 12 months
2. During Return to DOHA RP Must be Valid (If Expired She cant not Enter in to Qatar)
3. When she decide to back to DOHA (after 6 month & before 12 month), you just go to Madinat Khalifa Immigration Office, Gate # 03, 1st Floor, take your original passport & QID, wife's Passport Copy with Visa Page & QID Copy + Pay Qr.500/- Fee by credit/debit card
4. Then take the "RETURN VISA" Copy (and it is valid for 30 days. your wife should return before 30 days or before expiry of RP which ever is earlier, ) 5.scan and send this RETURN VISA copy to your wife, she must keep with her during her travel, immigration department in your country, they may ask this copy some time...
All The Best for you to join with your wife soon.