In addition to your mother needing to have sufficient compensation to support the amount of the contribution to the Roth IRA, your mother's MAGI for the purpose would also need to not exceed the amount above which some part of the contribution would become an excess contribution. This sort of contribution essentially constitutes two independent things under the tax code, a gift from you to your mother and a contribution by your mother to her Roth IRA (no matter which of you actually makes the deposit).
As long as no part of the Roth IRA contribution is an excess contribution, there is no reporting requirement on your mother's income tax return. As long as you have made no other gifts to your mother during the year, the maximum permissible Roth IRA contribution made as a gift to your mother in a particular year will be under the gift tax exclusion for that year and no gift tax return will need to be filed.
