פירוש על קידושין 46:11

Second tithe produce is usually redeemed, the money is brought to Jerusalem and used there to buy food products. The issue here is a woman redeeming her husband’s second tithe. When one redeems one’s own second tithe, one is liable to pay an added fifth. But when redeems someone else’s second tithe, one need not add the fifth.
According to the first opinion, that of the sages, the woman essentially is like her husband, and therefore she must add the fifth. According to R. Meir she need not add the extra fifth.