No it is not mandatory in case she is paying than she can get an exemption for that amount.
If the co-owned property is sold, each co-owner has to offer the capital gain as applicable on his share of the building. It may be noted that the apportionment shall be made at the ‘sale consideration’ and ‘cost of acquisition’ level and not at the ‘net taxable capital gains’ level. So, in the case of long-term capital gains on sale of the jointly owned property, whether commercial or residential, each one of the co-owner shall be entitled to claim exemption under Section 54F,the conditions of not owning more than one residential house as prescribed under Section 54F for claiming exemption from long-term capital gains, shall also be considered for each of the co-owners and not for all the co-owners taken together. So each co-owner can get exemption upto there investment.