May a man treat a woman or a woman treat a man?

Narrated Rubai bint Mu'adh bin Afra:

We used to go for Military expeditions along with Allah's Apostle and provide the people with water, serve them and bring the dead and the wounded back to Medina.

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا بِشْرُ بْنُ الْمُفَضَّلِ، عَنْ خَالِدِ بْنِ ذَكْوَانَ، عَنْ رُبَيِّعَ بِنْتِ مُعَوِّذٍ ابْنِ عَفْرَاءَ، قَالَتْ كُنَّا نَغْزُو مَعَ رَسُولِ اللَّهِ ﷺ نَسْقِي الْقَوْمَ، وَنَخْدُمُهُمْ، وَنَرُدُّ الْقَتْلَى وَالْجَرْحَى إِلَى الْمَدِينَةِ.