My Personal Experience-If a person who does not believe on existence of God ,if he/She is in trouble,then take pity on him/her,love him,be kind for him/her,pray for him/her, but do not help him/her till he/she does not start believing on God.Otherwise you may be in between the person who is being punished by God due to the Sins he/she has done in past.It shall be just like ,one is punching on wall and you put your palm in between.So you will be injured.And you shall have to bear the punishments of his/her bad deeds of past.
Off course God has capacity to bear the pain of somebody but you won't have.
And reversely If a believer of God is in trouble then try to help him/her as far as possible you can.