Praise be to Allaah.
It is OK for a woman to wash her husband if she is knows how to do that.
‘Ali (may Allaah be pleased with him) washed his wife Faatimah (may Allaah be pleased with her), and Asmaa bint ‘Umays washed her husband Abu Bakr al-Siddeeq (may Allaah be pleased with him).
Kitaab Majmoo’ Fataawa wa Maqaalaat Mutanawwi’ah li Samaahat al-Shaykh ‘Abd al-‘Azeez ibn ‘Abd-Allaah ibn Baaz (may Allaah have mercy on him), vol. 13, p. 107.