It is makrooh to pray facing a person’s face

There is nothing wrong
with praying behind someone who is sitting if he gives you his back;
rather it is makrooh to pray facing his face, if that will distract the
one who is praying. 

Ibn Qudaamah (may Allah
have mercy on him) said in al-Mughni (2/40):

It is makrooh to pray
facing a person’s face, because ‘Umar disciplined people for doing so.
According to the hadeeth of ‘Aa’ishah, the Prophet (blessings and peace of
Allah be upon him) used to pray level with the middle of the bed whilst I
was lying down between him and the qiblah. When I needed something, I would
not want to get up facing him, so I would slip away. Agreed upon. End
quote. 

Al-Nawawi (may Allah have
mercy on him) said in al-Majmoo‘ (3/230): It is makrooh to pray with
a man or woman in front of you, facing him where you can see him. This was
regarded as makrooh by ‘Umar ibn al-Khattaab and ‘Uthmaan ibn ‘Affaan (may
Allah be pleased with them). And because it usually distracts one. It is
also makrooh to look at what will distract one, such as a garment with
decorations on it, or looking up towards the sky, and other things that are
mentioned in the saheeh hadeeths. Al-Bukhaari said in his Saheeh:
‘Uthmaan (may Allah be pleased with him) regarded it as nakrooh to face a
man when praying. Al-Bukhaari said: Rather that applies if one will be
distracted by that, but if he will not be distracted, Zayd ibn Thaabit said:
I do not mind; a man does not interrupt the prayer of another man.

End quote. 

With regard to one who is
sitting with his back towards the one who is praying, if he does not move
and distract the one who is praying, there is nothing wrong with praying
behind him, just as there is nothing wrong with praying facing one who is
sleeping, because of the report narrated by al-Bukhaari (382) and Muslim
(512) from ‘Aa’ishah, the wife of the Prophet (blessings and peace of Allah
be upon him), who said: I used to sleep in front of the Messenger of Allah
(blessings and peace of Allah be upon him), with my legs in the direction of
his qiblah. When he prostrated he would poke me and I would draw my legs up,
and when he stood up I would stretch them out. She said: There were no lamps
in the houses at that time. 

And Allah knows best.