For visa on arrival you need a return ticket which is within 30day of your travel, Immigration officer in India will ask this for your mom.

Tourist Visa (Upon and Before Arrival)

Citizens of the 80 approved countries can obtain a Tourist Visa upon arrival. This visa can also be issued online prior to travel.

Online Instructions
Sign up or Sign in using your e-mail and password.
Enter the request's details and attach all the required documents including:
Passport Information (Bio/Picture Page)
A Passport-Sized Photo
Airline Ticket (Qatar Airways passengers are not required to submit their ticket copy)
Hotel Confirmation Voucher / Relevant Documents for Place of Stay in Qatar
Follow all the necessary steps, pay the fees, and submit your request.

Additional Information
Applications can be made a maximum of 90 days or minimum of four (4) working days prior to travel.
The passport should be valid for at least six (6) months from the date of entry in Qatar.
It’s not possible to use this service while applying for children who are endorsed on their parents' passport.
You can upload the following additional documents to strengthen your application:
Documents showing investments of USD $8,000 at least.
Income tax papers showing an annual gross income of USD $8,000 at least.
Evidence of travel in the last five (5) years to any of the following countries: QATAR, Australia, Canada, New Zealand, Schengen countries, USA or UK.
Fees’ Payment can be made with Visa/MasterCard (Debit) or (Credit) only. To know the fees’ value and more details on the service, please visit this link.
Once the application is submitted, applicants will be contacted via e-mail within 48 hours with a response to their visa application.
For more information on the countries of which citizens can apply for this visa, and on the terms and conditions, kindly check this link.