Section 15. Relief in case of opposition on certain grounds.

In any suit instituted for dissolution of marriage, if the respondent opposes the relief sought on the ground, in case of such a suit instituted by a husband, of his adultery, cruelty, or desertion 1 *** or, in case of such a suit instituted by a wife, on the ground of 2[her adultery or cruelty or desertion], the Court may in such suit give to the respondent, on his or her application, the same relief to which he or she would have been entitled in case he or she had presented a petition seeking such relief, and the respondent shall be competent to give evidence of or relating to 3[such adultery, cruelty] or desertion.