That does not spoil his fast, and his fast is valid, because when this happens it is not by their choice, and they did not mean for it to reach their stomachs. I would like to take this opportunity to explain that things which break a person’s fast – namely intercourse, eating, drinking, etc. – only do so if three conditions are met:
1 – that he knows (the ruling). If he does not know the ruling then it does not break his fast, because Allaah says (interpretation of the meanings):
“And there is no sin on you concerning that in which you made a mistake, except in regard to what your hearts deliberately intend”
“Our Lord! Punish us not if we forget or fall into error”
And the Prophet (peace and blessings of Allaah be upon him) said: “My ummah is forgiven for mistakes and forgetfulness and for what they are forced to do.” The one who is ignorant is making a mistake, and if he knew he would not have done that, so if he does something that breaks the fast out of ignorance, then he is not to blame and his fast is complete and is valid, whether he was ignorant concerning the ruling or the time.
A similar case is when a person does something that breaks the fast, thinking that it does not break the fast, such as one who has cupping done, thinking that cupping does not break the fast. We would say to him, your fast is valid and you do not have to do anything. And there are other things which happen to a person not by his choice; so there is no blame on him and that does not break his fast, for the reasons we have mentioned.
In conclusion, the things that break the fast do not break it unless three conditions are met:
1 – he should know the ruling
2 – he should not have forgotten that he is fasting
3 – he should be doing that by choice.
And Allaah knows best.