To study graduation in India as you said one has to have a 10 +2 qualification or diploma qualification. That is true. Up to what level your wife studied in a foreign country? You have not mentioned that. Some foreign qualifications are considered equivalent to +2 and some are considered equivalent to diplomas.
As per the Association of Indian Universities( AIU), the following courses are considered equivalent to the +2 stage qualification of an Indian Board. This list will be amended from time to. Foreigners who are having one of these qualifications can get admitted to UG courses.
a. 12 years Baccalaureate Certificate of Afghanistan, France, Iraq and Tunisia.
b. 12 years General Secondary Education Certificate of theMinistry of Education – Bahrain, Egypt, Iran, Jordan, Kuwait, Oman, Saudi Arabia, United Arab Emirates and other Gulf countries.
c. GCE/IGCSE examinations of the approved British Examining Bodies in UK, Singapore, Hong Kong and Mauritius with passes in 5 subjects at 'Ordinary Level' and 2 subjects at 'AdvancedLevel'. For admission to professional courses- medical /engineering Advanced Level Passes in Physical, Chemistry, Biology, Mathematics and English are essential besides 5 (five)passes at GCE 'Ordinary Level'.
d. High School Diploma from accredited School of USA.
e. Intermediate/Senior Secondary/Higher Secondary of the approved Boards in Pakistan, Bangladesh and Nepal.
f. 12 years Secondary stage qualification of National ExaminationBoard/Council in of the Ministry of Education – Kenya, Nigeria, Ethiopia, Sudan, Uganda and Tanzania.
g. 12 years secondary stage Examination conducted by various provinces in Australia and Canada.
h. GCE 'Ordinary and Advanced' level examination of the Department of Examination, Govt. of Sri Lanka
i. 12- year STPM examination of Malaysia.
j. 12- year Secondary Stage qualification of Indonesia, Japan, Korea, Socialist Republic of Vietnam and the Yemen Arab Republic.
k. 'Abitur' of Germany.
l. 12- year High School Diploma awarded by the accredited American School /Foreign Schools in other countries following American pattern of education.
m. Any other qualification determined as equivalent by AIU from time to time .
The above list is taken from the website of the AIU https://www.aiu.ac.in/. You can also refer to the same.
IGNOU is offering various courses to foreign students. You can check this page for the details.