Sorry you can't apply the Desktop CD/Download program purchase to the Online version. They are separate prices and work different. Did he file and pay from Online? If not he can move his return to the Desktop program. I can tell you how.
You should be able to install the program on Windows 7. What problem did you have? Here are some links
Windows System Requirements says you can install on Windows 7
https://ttlc.intuit.com/community/system-requirements/help/minimum-system-requirements-for-turbotax-...
Installing on Windows 7 Information
https://ttlc.intuit.com/community/tax-topics/help/windows-7-end-of-support-impact-for-turbotax/00/85...
And see
https://ttlc.intuit.com/community/tax-topics/help/what-if-i-don-t-want-to-upgrade-from-windows-7/00/...
